Don't spend any money on food-like outside the theaters, or the stateroom celebrations you can pre-order. There is so much to eat on board I can't imagine ever being hungry. You can always get food and bring it in to watch a movie. The same goes for soda-get it at the deck 9 drink station (I think it costs $ at the bars?).
Give your kids a pre-determined spending limit, and when it's gone, it's gone. We do this with our kids. They have to pay for everything they want (we pay for the excursions and major tips). It's amazing how smoothies are less desirable when they have to pay for them, and they really think about what souveniers they want before they get them.
We also ckeck our stateroom account each night. Sometimes if you know how much you are spending, it's easier to slow down.