I went with LDE Neon for both my kids. It's also what I use, I really like the KDE ecosystem, very polished experience overall.

Also it's similar enough to Windows that they don't feel completely out of touch on the school computers.

Killer app is Krita for the older one, even got him a cheap Wacom later on.

Otherwise school work, native Linux games, and YouTube. That last one with the younger one I have to keep an eye on. Honestly thinking about blocking access, we'll see.

Both like gaming, so I set up wine for them on my machine with separate accounts. Learning about email, downloads, files and folders by installing Skyrim mods (using kid friendly settings on Nexus), also an exercise in managing frustration...

Anyway everyone will be different, just set up something they'll enjoy. Already for a kid to know Linux exists is a head start.