Does my toddler need vitamins?
Not necessarily. Some studies show that if young children eat a variety of foods, they will get a balanced diet over weeks and months, not days. Children's vitamins lack large doses of nutrients, anyway, because nutrients have a bad taste that even lots of sugar can't mask. One-a-day children's vitamins, chewable or liquid, can be an insurance policy, though.
That said, be sure not to give your baby vitamins containing too much iron. The Dietary Reference Intake for children aged 6 months to 4 years is 10 milligrams a day, so that's the most I recommend. But since iron overdose (though usually from swallowing an adult's pills) is one of the most common causes of childhood poisoning, I prefer vitamins with no iron at all.