This might seem a bit “too much”, but as a linux user for 20 years, and someone that also is preparing a computer for my niece (she is 4 right now, but I’m going to give it to my brother so that he can decide when the time is right for her), I have to say you should look in to the new Commodore OS (yes; it is the Commodore!). It’s a large OS, but it has everything. What you want to do is hide everything that shouldn’t be accessible to your children, and that’s as easy as right-clicking on “Applications” and unmark any app that shouldn’t be visible (this is easy in any distro, but), it also comes with a few “education” apps (see image), that are very good as an introduction to computing. When they get older, you simply mark the apps you want them to have access to. I’ve used this dist for about 2 month now, and it is extremely stable. There are also other nice things that can help keep things a bit “fun”, like the “Cube” (check my profile header for how it can look, it’s from my current setup). The cube (in my opinion) is a great visual as to how, for instance, directories and such work; if you have let’s say 4 apps open on your desktop, going in to cube mode you can see how they all are as “layers”, this just makes sense (to me, at least).

I think you should check it out!