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.
Be a parent, and be present when they are using it. You don't have to watch constantly, but at least be nearby. Teach them basic digital safety, literacy, and privacy skills.
You cant control all the devices they have access to, so you have to give them to knowledge and tools to be safe.