@irontoenail - The best vitamin supplements for kids are going to be fairly mild though. The thing is, it's not a matter of "the more, the better" after a certain point. Vitamins can be dangerous if they are given in huge doses and some of them don't leave the body very quickly, so they can build up over time.
Iron is a good example. Too much of it can be very dangerous. A mild vitamin supplement probably isn't going to do any harm, but if you are concerned about their eating habits I think it's probably better to just get a blood test to make sure there's a problem before you try to treat it.