If someone other than the child can waive those rights, then they never had rights[0] in the first place.
0: Or, more pedantically, the rights that are inalienable to them by virtue of being a person were never not violated. Or, more comprehendibly, they never had acknowledgement and enforcement[1] of those rights by the legal system.
1: I assume "children do have [acknowledgement and enforcement of their preexisting] rights" is what dgzl intended to convey.
