I think it is a good idea to steer away from foods because no matter what you do, there is always potential for an issue.
Our elementary does not allow home made goods. EVERYTHING much be tagged with an ingredient label from a bakery, etc.
There will be a parent who heard from their Little Johnny that he didn't get a birthday treat from your kid because of an allergy OR he did get a treat, shouldn't have and not the parent is pissed. It is a no win situation.
Also, I am in the classroom a lot for these birthday treats and I have children on my list with allergies and I have to say no to a treat for them EVEN when they try to convince me they eat that at home. Also, it is so messy because it is done at recess and most of the time, over 1/2 the food is tossed in the trash.
What I have seen that worked well in a couple of classrooms was a special non-edible treat such as a book, some cute light up toy, pencils and erasers, glow bracelets, etc. I am not talking pricey treats... go to the Dollar Store and you can find cute little treats that kids love.