<<Disclaimer: Verify this information before applying it to your situation.>>

The best way to handle it is to keep a supply of acceptable treats at the
school.  Most of the things I can think of require refrigeration or
freezing for longterm storage--but, if you are in the U.S., you can force
the school to find the freezer space if parties or any kind of classroom
treats are permitted during school hours.

The archives of the Lowcarb Support List at MAELSTROM are full of great
lowcarb recipes--nearly all of which would be gluten-free or easily
adapted.  Lynne's chocolate in all its variations, pemmican candy, spiced
nuts, coconut cake, maybe even cheesecake (admittedly not for all
children)--and that's just for starters.  If you need something else, there
are several different lowcarb lists with very creative cooks who would
probably be able to help solve the problem.

I think it's appalling that they didn't have apples--but it's also
appalling to think that a young child would be watching everyone else feast
while eating only an apple.  She needs treats that are so outrageously
delicious that the other kids are jealous of her.

Barbara, who thinks that the key to compliance on any special diet is
having legal treats so that the diet is very special indeed