Really, there should be checks, but idk why ISPs aren't the place for it. When you have an account, you should be able to associate account levels and have decent parental controls for content filtering, web traffic reports, Mac filtering. Make it opt in for adult content. Really think about what children have access to, and the crap tools parents have to manage access.
Of course phones should also build that in much better, but I don't think it's sunk in yet.
It would be the best option in my opinion, because there isn't enough tech literacy in parents to have it be opt out.
Heck, web reports would be enough for most.