They don’t need to be protected from nudity in and of itself. However if they are talking to an adult or a stranger randomly messages them, it would be nice if

They don’t need to be protected from nudity in and of itself. However if they are talking to an adult or a stranger randomly messages them, it would be nice if the device blurs pornographic imagery. If they are looking for porn themselves (e.g. are of the age to care for such things), they can download it themselves via Safari. But generally speaking there’s no need to trade this stuff on iMessage.