You REALLY need to ask your pediatrician before starting any vitamin regimen. Their bodies can only process so many mg of certain vitamins/day, and even children's vitamins are formulated for older children vs. toddlers.
Our pediatrician has both of our kids (ages 2 and 4) on 1/2 Flintstone vitamin/day. Both of our kids are at the top of the growth charts, so it's not because they're small and have different requirements.
PLEASE be VERY cautious when selecting a vitamin. Most are NOT FDA approved - especially those you purchase via an independent vendor or over the internet. It means there's no validation of quality of ingredients, or purity. It doesn't mean they're not safe, there's just no one monitoring the products to ensure your child's safety. You can go to FDA.gov and search to see what products are regulated.