Hi,
I myself have Asthma, and so do my kids (although much milder)

Here's what I do :

1. Bring them into a dimly lit room, but enough light so you can see
them breathing properly.

2. A cool rag for the forehead helps to calm them down.

3. Now the most important part: Pursed Lip Breathing
Have them breathe in through their nose slowly and blow out slowly
through puckered lips as if they were blowing to cool off a bowl of
hot soup.

Reinforce that they try and slow their breathing down with each breath
back to their normal rate and rhythm. This might take a few minutes.

This has spared us from many trips to the ER.

p.s. I would find out what triggers their attacks, and maybe there is
some maitainence meds they could be on, ask your Doctor.

Good luck and God bless!