What is legally required is different from what you're duty bound to do. If you adopt full responsibility for a child then yes, mother and father are duty bound to even give an organ if need be. And certainly to sacrifice themselves to save their child. That's why on the Titanic it was "women and children first" the men were duty bound to sacrifice their own lives if necessary. It wouldn't have been illegal for a man to jump on a life boat, but it would have been immoral