For the app side of things, technically you can build a Linux install that you manage remotely and the kid doesn’t get root access. You can use SELinux to restrict what the kids can do, or even stop a good chunk of code execution by making all the folders they can write to noexec. There will always be the problem of interpreters like Perl/Python just inherently existing on pretty much every system, but if your kid is that far they’re probably close to just installing their own distro anyway.
You can set up some rsyslog or similar log shipping to log every app being accessed to a central database. You can monitor network traffic both via DNS (others mentionned PiHole), you can also set up a transparent proxy server so you can log not just the domains but the contents of all traffic including HTTPS, and potentially block based on keywords or just passively log all activity and act on it after the fact. Knowing you’re monitored is a decent deterrent in itself.
Depending on your coding skills, screen time can be pretty easy to monitor and configure via SSH. You can configure it like it’s a server and use tools like Ansible or Puppet to make the computer automatically pull your scripts and policies.
All of those things involve a certain amount of work though, it’s understandable to want to stick with known tools that do the job well. I think the community generally don’t like these kinds of tools so very few people make them. I’m also somewhat on the side of educating kids about the risks rather than bubblewrap the tech, but I imagine these days we’re talking very young kids going online for YouTube Kids and whatnot. I’ve personally had unrestricted access to the Internet since ~7 ish and already had Mandrake 9 installed when I was like 9-10, and probably contributed to my self teaching sysadmin and code very early on and had a web dev job when I was 17. I also was definitely exposed to some sketchy adult stuff as a side effect of pirating all sorts of enterprise software to play with, but I think my mom did a good job of teaching me what it was and grossed me out of it so much that unlike my peers I just didn’t feel like seeking porn or anything until I was well past the age of legally going to those sites.