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.
As a prior child who now has nearly 30 years of IT experience, that's adorable. 10 minutes of research and you have been defeated.
Anything parents do just slows down a determined child. You can build a faraday cage for the pc, airgap it, install an obscure operating system that doesn't know what a network or a flash drive is... and they are just going to go over to their friend's house to use theirs.
Just be a decent parent, and for gods sake don't be overbearing. That's all you need.