This will greatly depend on the individual child, but the general answer is no. Nothing has to be work underneath to protect the skin, even in the more delicate areas. For children that have sensitive skin or get rashes easy, they may want to wear something underneath to protect them. Some purchasers said that a bathing suit may be worn underneath these suits, especially if it will be taken off when the child gets out of the water while still at the pool or beach. They would already be wearing something and not have to go change.