Some icecreams are not gluten free as they have glucose syrup in them that is derived from wheat....check labels.
Basco make quite a few good mixes for cakes that are gluten free.
I did a gluten free party for our LO for his 1st.
One thing that went down well was "traffic lights". I made mashed avocado as one filling, a tomato based dip as another and a carrot dip as another then I spread these on savoury crepes and rolled them up and cut them so they made spiral circles.......then arranged them as traffic lights. They were healthy and kinda interesting and the grownups ate them too!
You can easily check labels for gluten free icing sugar for cakes and most recipies work pretty well if you just use a good quality GF free flour instead of regular flour. Don't make cakes well in advance though as they go stale very fast compared to normal cakes.
Jelly is great too and a fun (usually GF) food.
Some chocolate has gluten so check.......
On the upside I took my LO to a party recently and although there was not much that was GF he didn't really try and eat anything he shouldn't anyway.
Good luck and I think it's wonderful you are trying to do something special for these kids.