If you want to go with traditional, Patties make GF party pies and sausage rolls. I would do them specifically for the GF kids as they are expensive! You can make honey joys, just check the the corn flakes are GF (the GF ones are generally in the health food section at the supermarket). As someone else said, you can buy GF frankfurts. A fruit platter is always good, so that you feel like you offered something other than crap!!

Allen's make GF lollies, the bags are specially marked. You could make goody bags with a few lollies and some little toys from a $2 shop?

My mum is coeliac and I bake a GF sponge as our 'celebration cake' (either that or sticky date pudding...mmmm!!). Happy to send you the recipe if you like? It's yummy and not hard if you like to bake!