I would look at the Atomic spins from Fedora or other immutable distro. Your Parents can be separated from the OS while being able to install/uninstall user software as they like without a problem. The OS can update itself in the background without them even knowing about it. The Budgie version is simple to use with an easy to get used desktop. It also offers just enough customization to make most people happy. The atomic Cosmic spin might also be a possibility also.