K.P.
Give what you can, but not more than you can afford. This is a tough year for everyone. Last year we gave the "lead teacher" the equivalent of one week's tuition and the aides each $50 in cash. This year, we are literally cutting that in half b/c we need to.
Remember that they do get paid and that your holiday "bonus" is really just a thank you! If you can afford to give some extra cash, then a nice card with an appropriate amount would undoubtedly be appreciated. If you are watching your budget, offer to buy them lunch one day. Bring in a few take-out menus and take their "order" and then have it delivered. Just make sure that you let them know ahead of time so they don't pack their lunch.