L.D.
You are so considerate and thoughtful! My son is also on the diet and it has helped a lot. First, check with the mom's of these children because they may already have some gluten-free casein-free (GFCF) cupcakes in the freezer waiting to take to the next birthday party or they may have already purchased the mix or all the ingredients to make up a batch just before the party. And they probably already have planned to bring an assortment of safe snacks that for their child to eat at the party.
If they don't have any plans on bringing their own legal cupcakes, then you can always purchase the Cherrybrook Farms cake and frosting mix from Whole Foods (make sure that it says GFCF on the front!) and make a batch for them if you are so inclined. They are easy to make and are very yummy. And you can also have your bakery birthday cake as well for the other kids who are not on the diet.
Also, over in Woodland Hills is a place called GF Meals (part of Your Dinner Secret) that makes everything that a child on the diet could ever want and everything that we have tried so far have been just awesome. Their website is www.gfmeals.com and you can order on-line and they can ship it to you.
As far as snacks go, here's a link from www.talkaboutcuringautism.org for a list of GFCF food and snack options that can be purchased at most grocery stores:
http://www.talkaboutcuringautism.org/gfcf-diet/gfcf-food-...
Again, I want to commend you for being so thoughtful and also for just being a good friend and setting an example for your children about what kindness, tolerance and inclusion are all about. Keep it up!