I'd probably give them Linux Mint.

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.

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

You probably want EndlessOS

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.

Puppy?

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.

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.