The child will be under parental control while using the PC
In the near future (in 3 months), he plans to purchase the F-Secure Safe package for his godson.
I suggest trying Kaspersky Safe Kids Free. Parents can use the system too. If they do, they can simply exit Safe Kids. The free version provides core protection features and is sufficient for your situation. It blocks explicit content on supported search engines and YouTube on supported browsers. It also uses heuristics to block inappropriate content. Safe Kids includes categories and custom blocking, enabling you to allow excluded websites only. It is a separate program and not just an extension or module in a suite, which allows it to work robustly, even with a VPN connection. I have been using the Premium version on Windows systems for years, and it has been working light and flawlessly with no issues.
[Safe Kids recommendation is not just a suggestion but a proven option that I have been testing or using for years. I chose Safe Kids (tested a few) because it is stable, offers excellent protection, and has competitive pricing compared to other popular options. You can use one account for all your devices. Safe Kids is reliable for protecting teenagers as well. It is one of the few that works effectively with services like VPNs. If I remember correctly, it will give you the option to test the premium version for free for 7 days when you create an account. This allows you to determine if the free or premium version meets your current and future needs.]
I think using browsers and DNS-level protection is enough for 6-year-olds. You can turn on Safe Search on browsers and Restricted Mode on YouTube. Use ControlD Family Friendly DNS Free on the system. It locks Safe Search on browsers and Restricted Mode on YouTube, so kids can’t turn them off.
F-Secure needs a Windows Standard Account to prevent kids from bypassing it. However, it does not work or block with a VPN connection, like most. The software has basic features for paid parental control software.