When I was a kid I remember being confused at what the installation questions were.

If I were my own parent, I’d explain how the OS can sort itself (FHS, Windows, macOS). This gives confidence for installing and inspecting software.

Next, I’d explain how drives work and how they’re represented in different systems. That means partitions and formatting too.

That would then take me to explaining what’s involved in OS installs, erasing everything, dual booting, retaining personal files, etc.

DebianJr is the easiest pre-packaged distro for kids 7-12.

Under 12, I would keep it fully offline(remove NIC or blacklist MAC address.

Install a local wikipedia instance (or simple-wikipedia) for reference, and give them thumb drives/DVDs for media.

For the fully libre start to life, install Trisquel GNU/Linux and use the DebJr package list to install required software.

I'd go raspberry pi for kids - gpio projects are fun and linking computer to physical world.
The newer ones are a bit pricey for what they are though.

Every distro is essentially the same it's not the question you want to be asking

You probably want EndlessOS

When i was 9 my sister gave me a laptop she put ubuntu on and it pretty much worked for me. I have a pretty good sense for computers and maths so that probably helped. And on another note, no i didnt get trauma from unrestricted access to the internet because i never searched for stuff like that. The first time I started seeing(and thus avoiding) gore/shock videos was when my classmates also got devices. I think if you teach your kid well they will learn to avoid these things but be carefull because other parents(especially today) dont teach their kids about safety. Your milage may vary, im not a parent.

My wife is not a fan of the idea, however .

Divorce!

Jokes aside, Edubuntu should cover you parental controls and Education tools needs. And since it comes with Gnome by default the UI should comfortable to your children. I suggest diving into it if you need more something sprecific. Good luck!

PS: There are some good articles flying around about how to convert your loved ones to linux. I have one measly convert and my advice is to show them how can linux solve their problems.

Kubuntu + adguard DNS and you're done. Good looking desktop which is fun to configure for a kid. Dolphin file manager can show the terminal in a tab, which is good to learn. Nothing compares.