"Mommy," Penguin's voice is hushed, tinged with a note of horror, "Some crayons are made out of...soybeans."
Er.  Um.  Yes, yes, I'd heard about that. 
"But what about kids who are allergic?!?" 
Er...well, depending on how bad their allergy is, their parents have to research the crayons and art supplies, and sometimes provide safe ones for their kids preschool or kindergarten class. 
"Could the kids wear gloves?"
If they're old enough not to eat the crayons, and be trusted to wear them. 
"Thats why they just buy crayons for everybody?"
Yes.  Kindergarteners and preschoolers might not keep their own crayons to themselves.
"What about kids my age?  Would they just bring their own?"
Probably.  Or they could be trusted to wear gloves and not eat the crayons.  I hope. 
She giggles.  Then frowns.  "What about the ones who don't want to look like dorks?"