I disagree. Assuming it's their own device, set up automated backup, but let them trash the OS. PCs/laptops are (still) pretty much un-brickable at software level. If they do, re-installing & fishing around the backup will be a learning experience, and the fact you trusted them "this is YOUR device" will be a positive ownership experience that is getting rare nowdays...

* Linux live USBs can be a nice safe intro to such power? Give them 2-3 with differnt-looking distros (even if cosmetic ;-), tell them "try this for a day, if you decide which one you like it you can install it permanently". Hmm, I should try this with my kids!