You will still have false positives. You will either have false positives in that children will slip through, which the FTC is not a fan of. Or you will have false-positives in that adults will be unfairly restricted (which everyone else should have a problem with).

Even in that scenario, content filtering is more feasible and practical than detecting and banning children.

But, pushing that problem aside as well, sure. Assuming that you didn't have false positives, and assuming that you didn't have to violate anyone's privacy, and assuming that the solution wasn't obscenely expensive, that theoretical solution would be very helpful. It would basically solve the entire problem, for pretty much everyone.

But that theoretical solution doesn't exist.

If we're willing to make those assumptions, I would also like a special gun that only shoots bad people and doesn't work in schools or churches. And it would be nice to give law enforcement with warrants access to lawful information without requiring encryption backdoors. Of course, we'll also need to implement the new training regiment that gets rid of any corrupt prosecutors or officers.

I'll happily posit things for the sake of discussion, but there's a point where I'm suspending so much reality that the discussion just breaks down. I can't stop looking at the proposed solution and saying, "yeah but this thing you're talking about doesn't work." I guess I'm just not sure what you're getting at.