Debian, XFCE. It'll be easy to use through point-and-click, decent hardware support, and the local nerds will almost certainly be able to reverse any software issues.

Kids commonly enjoy drawing, so it's likely a good idea to install Krita, which I'm not so sure is in the repos and might require a bit of imposition of hands.

Libreoffice will be installed by default, one might want to switch it out for something else for whatever reason.