Well, worst case scenario, they die. I just checked and there's been one death due to iron poisoning since 1998. But yes, there are quite a few young children who get sick from too much iron, usually because they think they're eating candy. Imagine if we fortified beer with iron. I'm sure we'd see a lot more adult cases of iron poisoning in that case.

This may sound like splitting hairs, but I would classify minerals separately from vitamins. It's easier to develop a mineral toxicity.