Use regular old rubber bands to help control spills.
Secure two or three rubber bands around a cup or glass, one above the other. The bands provide traction around the perspiring cup and help prevent it from slipping through your child's hands.
P.S. That is a rubber band ball in the pic, one of BestBuys' favorites to make as a boy. Just wrap rubber bands around a nickel and see how big you can get it to be! Cheap fun for the kids! But be warned, if a brother throws one at his sister, it will really hurt...take my word for it...