C.K.
I second the food idea--something that can be shared. Most people don't need another mug or knick-knack.
I also think a gift card to Target or Micheal's would be most appreciated. The budgets at most centers are tight, and they could purchase stuff they need.
I would have the card read something like, "Thank you to Mary, Sue, and Beth--and everyone else--at XYZ Center for taking such great care of my kids." There may have been people who did stuff here and there with your kids, and you don't want to hurt their feelings.