Obligations to children

It seems generally accepted in Libertarianism that you have a moral obligation to your own child but why? How long does it last? How is this idea incorporated into the NAP?

Often times when faced with a scenario like “Let’s say you have a pile of bread that’s about to go bad and someone is standing right next to you who only needs a bite.” The answer is “You are not morally obligated to give it to him. You may be a d*ck but you’re not morally obligated.”

You could say by bringing them into the world you formed an obligation but why?

It just seems like the topic of children is its own island of logic when it comes to Libertarianism that isn’t applied anywhere else.