Because adults remain children. As in, their parent’s kids and therefore property. [edit: I should mention also property of the state beyond that] It’s less explicit in US I guess but in some places that’s very blunt - if you don’t support your parents enough you can be sued for abuse. And there are situations where an adult in us has been declared too irresponsible and forced into conversion camps by parents in the US. It’s insane, yes, and if you’re lucky enough this might be entirely invisible to you. But if you’re gray or trans or autistic and get a but unlucky this can become a very harsh reality.

Protect the children refers to a type of property, not a type of human.