Just recently I started giving Kellan multivitamins. They are just like gummy candy and he loves them - probably a little too much. I bought the vitamins because his picky eating habits made me wonder if he gets the nutrition he needs. Turns out, it's very likely he does despite his um, discerning palate.
According to the American Academy of Pediatrics, most children don't need vitamin supplements. Children store nutrients so even if they've spent the day barely touching a vegetable, kids can still benefit from your efforts to feed them a balanced diet throughout the week. Also, many foods are fortified, so even if your child will only eat Cheerios and milk one day, he will likely get all the vitamins and calcium he needs. One vitamin that doctors find children are seriously lacking is vitamin D, but the problem is easily remedied by drinking plenty of fortified milk.
Giving your child a multivitamin certainly doesn't hurt. Just make sure it's formulated for your child's age and that you keep it out of their reach. I've been especially careful about that since the vitamins I bought look just like candy. In fact, after paying close attention to what he does eat, I realized over the course of a week, he does eat more foods than I initially thought. Maybe I was worrying over nothing.