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.
Since it's a laptop, I would set up something like NextDNS (what I often use, but you can also use Cloudflare, ControlD, etc.) in order to block advertisements and unwanted content. Ideally use a Firefox-based browser as then you can use uBlock Origin (+ they are often less memory intensive!) to block additional stuff. Others have already suggested supervision and setting a different user account with fewer privileges. I would highly encourage using a solid Linux distro, as those break less often (especially when the kid becomes / is older and starts going to sketchier websites), and in the modern era they tend to work better too, particularly on older hardware! KDE Plasma and GNOME are both solid desktops available in a variety of distros, or you could go with the reliable Mint w/ Cinnamon.