If a "right" can be signed away, it's not really a right. I recall reading about a case where a woman agreed, in writing, to waive her right to maternity leave under FMLA. She then changed her mind, and the employer sued and lost on the grounds that rights cannot be signed away.

Looked at from that lens, you're arguing that a child's rights can be signed away. My argument is that therefore they aren't rights.