Mineral oil is not a terrible choice for pediatric constipation.  In fact
there is a commercial product out there called Lansoyl that is mineral
oil-based.  The main concerns with mineral oil is accidental aspiration
(into the lungs) and interactions with medications and food (reduce
absorption of fat-soluble vitamins for example).  Have you tried glycerin
suppositories?  They're safe, but obviously not something your young
patients would enjoy on a regular basis.