Obviously, I know its impossible to completely child-proof a PC, but theoretically, what steps would you take? Assume its a laptop with internet access, meaning you'd want local solutions and something like a pi-hole wouldn't work well.
I would personally assume, you'd want to:
A) discourage internet exploration by offering ready-to-use options for most needs, such as thumbnails/bookmarks for common websites.
B) Implement some sort of blacklist for common or easy-to-stumble-upon negative content. Maybe something as simple as a Ublock filter?
C) Robust monitoring tools, although I have no idea what.
Edit: To be clear, I know education is the number one tool, but let's say a tech illiterate friend is asking for help. What might you set up for them, or recommend they use - tools to assist in parenting, not replace it.
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.