Last year when my daughter was in Kindergarten I sent bags/gifts for the kids every holiday. Easter, Valentines, St. Patrick's day - you name it! I was also an art docent in the class, so at the end of the year I gave all the kids a little tiny paint set as a gift.
This year on the other hand, I haven't been to the class that often so I hardly know the kids; I'm not crazy about the teacher and I really don't seem to be that into the whole gift thing anymore. It's too time consuming, too stressful (always trying to out-do myself) and I end up spending too much money.
If you want to treat the kids to a gift, send cupcakes. Cupcakes are cheap, quick and a real crowd pleaser. The kids LOVE cupcakes. Anything else you send home is just going to get thrown away anyway.