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.
My setup has basic protections to stop them stumbling into things they shouldn't be looking at. It's intended as a bumper, not a barrier. I know I won't be able to make it impenetrable without also making it almost unusable. Even then, that doesn't solve things, if they are around a friend's etc.
Instead, I talk through things with them, in an age appropriate manner. If you can get a continuous conversation going then you don't need to lock things down hard. It might get awkward and embarrassing for you, but it's far more effective in the long run.