For your computer: https://help.ubuntu.com/stable/ubuntu-help/parental-controls.html.en

For the Phone I’d create a second profile on the phone and only install the apps there which I’d allow my children to use.

I’m still looking for a solution after https://keepandroidopen.org/

About 50% of my apps are from F-Droid (the others are specific local apps and especially bank apps, which is difficult …)

This is me!