To argue that children were not historically considered property requires a level of hair splitting that will necessarily be able to be turned right back around and used to argue that slaves and serfs in other contexts were not property.
>affecting people's understanding of the boundaries of rights and responsibilities around them.
Civilizations in ancient southern generally outlawed gratuitous violence toward enslaved or indentured people and/or obliged their owners to feed and cloth them. Does that make slaves not property?
In medieval and early modern central Europe landowners were legally responsible for productively working their land or a subset thereof (this was a large part of the legal doctrine used by the English settlers to justify taking of native lands). Does that make land not property?
(obviously the specifics of the above two examples vary by culture and exact time period but that's not the point.)
I'd be interested to see you cite a preindustrial civilization where kids are treated as second class citizens rather than de-facto property of either the individual or the tribe/state/other.