What can we do about my 4-year-old's thick, black toenails?
While toenails can become thickened for various reasons (such as fungal infection, psoriasis, etc.), your observation that the same changes are seen in your husband is important. You didn't mention the ethnic background of these loved ones, but sometimes the nails of individuals with darkly pigmented skin can darken and thicken as a normal variant. There is also a genetic disorder called "pachyonychia congenita" that causes thickened nails, thickened skin on the palms and soles, and white patches inside the cheeks.
Aside from fungal nail infections, most of these nail changes are difficult to treat, but there are some therapies that might help. And although some would consider treatment to be "cosmetic," these nail changes could make a young child self-conscious and could even affect walking or running. Have your daughter evaluated by a dermatologist or podiatrist (foot doctor), so you can get to the root of the cause and learn about possible treatments.