What would cause thick toenails in a toddler? Is there anything to do to get rid of it?

Congenital. Actually 99% of thickened nails result from a birth defect, which may show up as the child ages. There is almost always a family history of such things. The fungus mif there, is always the result of, not the cause of the problem. Thus antifungal therapies almost never cause it to disa[ppear. There is no known cure for "horn" nails.But they are cosmetic problems only.
Fungal infection. It is a sign of fungal infection and it is also happening in gut. Use epsom salt soak daily for 3 months. Locally apply antifungal cream daily. Orally give zinc, selenium and garlic supplements.