This kind of argument might hold water if there were an epidemic of 12 year olds aimlessly drifting across the countryside with no adults taking care of them. But there's not, and you're not really making any point.

If you are assuming a guardianship role over a child, however temporary, you have a responsibility to teach them things, full stop.

I've known several foster parents over my life who would be outraged at the implicated that they are somehow lesser to the children they raised and are raising.