Children do not owe their parents a single thing. Parents have children completely of their own volition, they usually enjoy the acts that creates the child. The child has their own life thrust upon them, and by the time they are aware of the fact that they had no choice in their creation, they can't do anything about it. Well, they can, but it would likely involve a large amount of pain and cause severe amounts of emotional trauma to those who know them. They are pressured into thinking that they have a gift, even if they don't want it, and that ending that would be a terrible thing to do - some even invoke religion, suggesting that God will send them to eternal punishment if they try to get rid of their 'gift.' I don't think anyone should be expected to repay someone for a gift that was forced upon them since before they were remotely conscious.
Imagine it this way - imagine that Person A are the Parents, and that person B is the child
A, because he wanted to, put a book in your house, unbeknownst to you. When you finally discover the book, you start to read it. The book is, in general, uneventful (kind of monotonous), though some parts in it made you laugh, other parts make you cry out of sadness, and some parts, yet, fill you with rage or disappointment. Person A, then, demands, not only that you keep the book which he wanted to give to you, but also that you pay him back for having given you the book in the first place. Though you enjoyed a few passages from it, or even if you enjoyed all of it, why should you have to pay him for slipping a book, that you didn't ask for, into your house?