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How would you safely and privately child-proof a PC?
Treat the laptop as a desktop and put it in a public space (ie: the livingroom). Meaning gets an external monitor, keyboard or some other reason that it becomes a stationary PC.
Then an adblocker or pihole will be fine. Then have a discussion with your kids, that there are some very ugly places (use death/gore/animals being hurt, etc) that are not suitable people their age (or even your age)... and this computer tried to monitor and block it.
This won't stop curious kids from being curious, but they will understand why the guardrails exist... and having it in a public space will avoid leaving them alone in their personal space with the untethered internet at their finger tips.