Maybe by not framing the issue as one of 'liability' and 'surveillance', and just teaching the kid right from wrong. As for how that's accomplished, that's the parents' responsibility.

They failed here, so the state will be stepping in for the next few years.

What should the parents' punishment be? How do you evaluate that the parents have reasonably met their duty? Are both parents equally culpable? What about divorcees with alternative live views? Why should this parental failure liability end at 18, what about 19 year olds, or those in their 30's? What about kids previously diagnosed with mental illness?

Those questions will make up the bulk of the defense's case, I imagine.