Linux

If I may ask, why is your wife not a fan of the idea?

What does your wife have against linux? All the porn pop up viruses are on windows, and getting your kid on apple is setting them up to be in credit card debt for the rest of their lives.

Even my senile ass grandparents use Linux and they don't complain about every little thing like they did with windows. My dad wanted a Mac but free so after hackintosh being too much learning curve for him I used some random Mac inspired configs from the internet for one of the Linux DEs that I've never personally used, also no more babysitting and virus induced full wipes.

Parental controls I would do at the router level because eventually kids will surpass you in computer skills. Or maybe they won't because of seo and ai articles taking over the web.

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.

dont use parental controls. Its fake, doesnt make sense, and limits learn oportunities. Any Linux works out. Linux Mint works great

If the child is really young, check out the sugar desktop environment. There is an official distro from sugarlabs and there is also a fedora spin (fedora soas)
If the computer should be a little more functional, the GNOME desktop or the Deepin desktop are good options imo

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.

You probably want EndlessOS

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

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?

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.