DIY weblog Dana Made It suggests that if your kids constantly grab new cup every time they want a sip of water to instead have a dedicated water cup for each kid with magnets attached so a glass of water is always at hand.
Simply use hot glue or superglue to attach two magnets to plastic cups. Two magnets as the curvature of the cup makes a single magnet less secure. If you're worried about dust collecting in the cups just make sure they are attached to your fridge upside-down.
TUTORIAL: fridge magnet cups [Dana Made It]