Before investing time you might also get a several distros on live USB sticks, boot each one up with the kid and parent, and see which one they like best before you install it. Make the kid part of the process.
Depending on the age of the child, make the computer discoverable. The full app store might be too much for younger children (mummy what's a flatpak?) But you might preload a "basic" and an "intermediate" app, eg Minecraft and scratch and then a (simple!!) Python IDE. And put them in discoverable, kid friendly places on the start menu.
Games. Lots of games. Both for their fun value and for teaching the motor skills of mouse and keyboard. Curiosity apps like Google earth.
For older kids, compatibility with their friends is important so make sure that things like LibreOffice, chat etc Just Work. No 13 year old wants to be the Odd Kid with the Bizarro Parent Computer. You can involve them in thinking about what it means to have choice in computing and to not just be a consumer, but they're still kids facing natural social pressures.
I could go on all day. One last point. After the thing is all set up and has been running for a few weeks, check in with the child and parent. What do they like? What do they not? And fix those issues.