The question I would ask is, if a man and a woman voluntarily get together and have children, how is it that the government can require the children to go to school where the teacher (from a different thread) might teach them all kinds of stuff that the parents don't agree with? After all, nobody other than the parents (except for God) had a hand in making the children. So, why are the children not more like their property than simply children?
I think calling the children the parents' property puts parents at the same level as the government, both fighting to be the rightful owner. Ironically, this is what makes the parents themselves disagree regarding their offspring, to the point of divorce and custody litigation.
If you study families and human history, you realise Mother and Father represent the interests of their own parents and lineage, which has little to do with individual interest in a libertarian sense. This is why in-laws are annoying and why we have political left and right.