Parental control/content blocker not working on MacBook Air

You won’t like this answer.

Skilled IT staff have issues with establishing and maintaining app whitelists, and maintaining locked-down client devices, for businesses and employees with substantial budgets for networking and related infrastructure.

Prisons have issues with controlling device access. Prisons.

Ponder what that means for your efforts here.

Every solution you can conceive—of short of incarceration, and maybe not even then—your kids can bypass.

Which leaves you to address the people issues directly, or to continue attempting technical solutions to what are fundamentally people issues.

Lock up the computers. Lock up the Wi-Fi in a schedule. Switch to a DNS service that claims content filtering. But even all of that will fail of course, if the kids are so inclined. And will fail on an allowance-scale budget.

Or you can work through the people issues with the people involved.

Because kids have agency.

PS: Given recent trends (e.g. Avast, a few years ago), I’d expect many of the security-related add-on apps will collect and re-sell metadata if not data from the users, too.