We have a legal limit to the amount an employee can accept at our school district. Something like $30 or so? Anyways, I'd check to see if the school your son goes to has a similar arrangement.
As an aide myself, I really don't expect gifts from my students or their families. I get paid to do my job, get sick days, personal days, health insurance, etc. A nice note or card from the family would be great, to know I was appreciated and thought of, but this is a tough economy, and many families are having a hard time providing for themselves as it is.
How about a homemade card, or a store bought one with a bit of extra love (glitter and stickers are a fav with my students) for the aides, and a small gift to the class, like some art supplies? We're always scrounging for supplies!