Alexa -- Taking extra vitamins and minerals CAN be a problem, but probably not just from taking an extra half of Flinstone vitamin.
For example, too much Vitamin A and D can have serious consequences. It is a fat soluble vitamin that is stored in your body. Vitamin C is water soluble and isn't stored, so having excess doesn't cause many problems.
EVERYTHING is toxic if taken in too high a quantity, even water.
It's critical to read dosing information on ALL bottles of medication, but most especially for little ones. Their bodies just don't process stuff like an adult's does.
Chalk it up to a lesson learned, candice, and move on from here.