From the Witherspoon Institute website:
The primary business of the state is justice. Because children cannot be autonomous, adult society has an obligation in justice to provide institutional structures that protect their most basic interests.
I was once a libertarian activist. I was on the platform committee of the national libertarian party twice in the late seventies. I used to give introductory talks about libertarianism in people’s homes when I was a graduate student.
I would begin these talks by describing the problems that contracts between consenting adults could solve. Often someone would ask, “What about children?” I would always admit that children posed a tough problem for libertarianism, but that we would deal with it in a more advanced lesson. Somehow the time for that more advanced lesson never came.