Because children are not fully capable of taking care of themselves, and so there is a norm that all adults (and older children) have a duty of helping and protecting them (even against themselves).
And also because if an adult harms a child, it is much more likely that the victim is innocent and didn’t “deserve” that harm than if the victim is an adult.
(and I don’t think “greater moral value” accurately describes the situation)
But is caring for yourself and your friends and family an instrumental value that helps you stay sane so that you can help others more efficiently, or is it a terminal value? It sure feels like a terminal value, and your “morality of self-care” sounds like a roundabout way of explaining why people care so much about it by making it instrumental.