Because they are so young, it will be easier for you to make the whole family gluten and casein free, exceptions given to the adults in the house. In doing so it also erases any possible accidental exposure, ie: kids sharing food, and any unhappiness ie: why can't i have that? Also by making it so that all three kids eat the same diet, if one of the others happens to have a milder version or just sensitive, you're doing that one a favor too.
Look for websites that are for food allergies and kids. There's plenty out there to help you find a support group or network, as well as merchants and bakery shops that specialize in food without gluten and casein (and bunches of other allergy foods too!)
Best of luck.