Obviously block access to internet nonsense. I just started using ControlD.com and it works like a charm. Or, if you have the time, make the PiHole and your Home Lab & Network a project that you two build together.
Make the command line kid friendly, like using bat instead of cat and LSD instead of ls and so on. Colors are stimulating and fun. Something like the hyper terminal is another idea.
I would appreciate someone building a faster alternative to the hyper terminal in C or Rust.
And then there's Nand to Tetris, which you should pick up together as well.
Regarding unsupervised time on the net: be strict. Regulate screen time. Hold the kid playfully accountable. No, 9 is not too young for anything. It's all about deliberate parenting & discipline.