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How would you safely and privately child-proof a PC?
Some quick solutions would be:
Use cloudflare's 1.1.1.3 and 1.0.0.3 DNS. At least this works on the device as long as they don't change the DNS on the device themselves.
https://developers.cloudflare.com/1.1.1.1/setup/#1111-for-families
Parental control settings on your router.