I couldn't find much about children's play sand in MEDLINE (>11 million
health-related articles) except these items about possible asbestos
contamination and one "hygienic" article in German:
     Bull Environ Contam Toxicol. 1990 Oct;45(4):486-94.

Examining play sand products for asbestos contamination.

Webber JS, Janulis R, Syrotynski S.

Wadsworth Center for Laboratories and Research, New York State Department of
Health, Albany 12201-0509.

PMID: 2177672
-----------------------
     N Engl J Med. 1987 Apr 2;316(14):882.

Asbestos in play sand.

Langer AM, Nolan RP.

Publication Types:
 a.. Letter to the editor
PMID: 3821832
------------------------------------

     N Engl J Med. 1986 Oct 2;315(14):891.

Asbestos in play sand.

Germine M.

Publication Types:
 a.. Letter to the editor
PMID: 3748107
-----------------

     Z Gesamte Hyg. 1980;26(11):790-5.

[Soil-hygienic relevance of play sand]

[Article in German]

Moller F, Romer R.

PMID: 7467510
-------------------------------
There was also one guideline for sand to be used on children's playgrounds
(presumably regular sand not play sand) which suggests possible contaminants
to consider:
     Offentl Gesundheitswes. 1990 Jan;53(1):7-15.

[Determining guidelines for metals in children's playgrounds in North
Rhine-Westphalia]

[Article in German]

Viereck L, Kramer M, Eikmann T, Konig W, Bertges WD, Gableske R, Krieger T,
Michels S, Exner M, Weber H.

Hygiene-Institut des Ruhrgebiets, Gelsenkirchen, Aachen.

In 1990, the State of North-Rhine-Westphalia established an ordinance on the
quality of playground soil and sand. This ordinance includes guideline
values for toxic metals (arsenic, lead, cadmium, chromium) in playground
soil not covered by vegetation and quality standards for sand to be applied
on playgrounds. Additionally guideline values were set for mercury, nickel
and thallium. The guideline values include two categories: guideline value I
represents the upper limits (95-percentiles) of the background levels of
toxic metals generally found in upper soil layers in the State of
North-Rhine-Westphalia. Guideline values II ("action levels") were selected
on the basis of toxicological considerations. In cases where concentrations
of metals above these guideline values are detected, immediate actions
(urgent redevelopment measures) are required. Quality standards for
playground sand were established to ensure that only noncontaminated sand is
applied for playgrounds.

Publication Types:
 a.. Guideline

PMID: 2150550
-----------------------------------
As you say, most publications concern occupational exposure to large
quantities of ordinary sand. Here are a couple of sample refs:
     Ann Occup Hyg. 2001 Apr;45(3):193-9.

Cohort mortality study of North American industrial sand workers. I.
Mortality from lung cancer, silicosis and other causes.

McDonald AD, McDonald JC, Rando RJ, Hughes JM, Weill H.

Department of Occupational and Environmental Medicine, National Heart and
Lung Institute, Imperial College School of Medicine, Dovehouse Street, SW3
6LY, London, UK.

BACKGROUND: In 1997 a Working Group of the International Agency for Research
on Cancer changed an earlier classification of crystalline silica as a human
carcinogen from Group 2A to Group 1, though commenting that the
carcinogenicity might vary with industrial circumstances and depend on
additional factors affecting biological activity, including the distribution
of its polymorphs.Objective: We aimed to determine whether pure quartz
exposure uncomplicated by the presence of other contaminating carcinogens,
as experienced by workers in the production of high-grade industrial sand,
was causally related to an increased risk of lung cancer. METHODS: A cohort
of 2670 men employed before 1980 for 3 years or more in one of nine North
American sand-producing plants and a large associated office complex was
selected for study. Of the cohort, 2644 (99%) were traced through 1994, and
certificated cause of death ascertained for 1025 (99%) of the 1039 men known
to have died. Standardised mortality ratios (SMRs) were calculated for the
main causes of death, using both US and state or provincial male mortality
rates for reference. FINDINGS: The main analyses of deaths, 20 or more years
after first employment against regional rates, gave the following SMRs: all
causes 109, lung cancer 139, other malignancies 98, non-malignant
respiratory disease 161, and nephritis/nephrosis 244. There were, in total,
37 deaths from silicosis or silico-tuberculosis, with one or more death at
least in all nine production plants. Analyses failed to show any relation
between lung cancer risk and duration of employment. The increased SMR for
lung cancer was wholly due to high rates in four plants in two states,
whereas no increase was found in the remainder of the cohort. CONCLUSION: In
the absence of information on smoking histories and risk in relation to
estimated exposure, the increased SMR for lung cancer (139), although
statistically significant, cannot be attributed confidently to crystalline
silica. An answer to the question of attributability must await the findings
of the nested case-control study, in which level of exposure and smoking
habits were ascertained for cases and matched controls. The strong
indication in this cohort of excess mortality from non-malignant renal
disease deserves further investigation.

Publication Types:
 a.. Multicenter Study

PMID: 11295142
-------------------------------
     Ann Occup Hyg. 2001 Apr;45(3):201-7.

Cohort mortality study of North American industrial sand workers. II.
Case-referent analysis of lung cancer and silicosis deaths.

Hughes JM, Weill H, Rando RJ, Shi R, McDonald AD, McDonald JC.

Department of Biostatistics, School of Public Health and Tropical Medicine,
Tulane University Medical Center, 1430 Tulane Avenue, New Orleans, LA 70112,
USA.

BACKGROUND: A cohort mortality study of 2670 men in nine North American
industrial sand plants resulted in 83 deaths from lung cancer 20 or more
years after hire (standardized mortality ratio 139) and 37 deaths from
silicosis (including seven from silico-tuberculosis). The lung cancer excess
was unrelated to duration of employment and not found in all
plants.Objectives: The primary aim was to determine whether lung cancer risk
among these employees was related to quantitative estimates of crystalline
silica exposure, after allowance for cigarette smoking. A secondary aim was
to do the same for silicosis mortality, partly as a means of validating the
estimated levels of exposure. METHODS: A nested case-referent study was
undertaken with cases matched with up to two controls on plant, age and date
of first employment from men who survived the case. Exposures were estimated
by linking work histories to a job-exposure matrix, undertaken separately.
Cigarette smoking information was obtained from medical records and other
sources, blind as to case-control status. Matched statistical analyses were
conducted using conditional logistic regression. FINDINGS: Odds ratios for
silicosis mortality were significantly related to cumulative silica
exposures and tended to a relationship with category of average crystalline
silica concentration, but inconsistently with length of employment. After
accounting for a strong effect of cigarette smoking, odds ratios for lung
cancer were related to cumulative crystalline silica exposure and to average
silica concentration, but not to length of employment. CONCLUSION: These
findings support a causal relationship between lung cancer and quartz
exposure after allowance for cigarette smoking, in the absence of
cristobalite or other known occupational carcinogens.

Publication Types:
 a.. Multicenter Study

PMID: 11295143
-----------------------
Sand and silica are everywhere:

     J Expo Anal Environ Epidemiol. 1997 Jul-Sep;7(3):377-84.

The regulation of crystalline silica: an industry perspective.

Elzea JM.

Thiele Kaolin Company, Sandersville, GA 31082, USA.

Silica is ubiquitous in the earth's crust. It occurs in trace to large
quantities in rocks and soil. Because it is so common, the regulation of
silica has affected a large number of industries, including the mining
industry and any industry that uses quartz in the manufacture of a products.
Mineral commodities that contain silica include diatomite, bentonite,
kaolinite, talc, pyrophyllite, sand and gravel, perlite, pumice, dimension
stone, and barite. Products that contain minerals, many of which are
associated with silica, include paint, paper, rubber, plastic,
pharmaceuticals, food, cement, plaster, cat litter, potting soil, plaster
board, and miscellaneous construction materials. In collaboration with some
agencies and academic centers, the silica industry is supporting research to
lower health risks and to improve the methods of detecting this common
material.

PMID: 9246599
-----------------
While I wouldn't encourage your toddler to wallow excessively in play sand
in such a way that large quantities of dust are kicked up and inhaled, or to
ingest it (eg. by not washing hands and eating while in the sandbox), I
wouldn't fret too much about it. Kids have played in regular old dirt and
beach sand and all sorts of other filthy things without getting cancer.
Silica exposure from other sources is likely to be as prevalent or worse as
whatever your child will encounter in play sand.  I'm sure some kind of
regulation forces the companies that distribute play sand to alert people to
possible carcinogens such as silica which are a natural constituent of sand
and the Earth in general. If there is reason to suppose that there are other
carcinogens such as asbestos or environmental industrial pollutant chemicals
in the sand (do they list the carcinogens?), then I would probably pick sand
from a different company or let the child just play in the dirt (which is
full of silica and probably pesticide residues, radioactive elements, and
all the other noxae that are inevitable on our planet).