Have you tried just creating a new project and just adding a single LED node? If that works, then your son has probably deleted a node or made some other change to the single project and not affected the entire installation.
If that doesn’t work, I would look in the folder where your XOD projects have been saved. Does it have a __lib__ folder? If so, what is inside that folder? If there is a xod folder inside, close your XOD application and rename that folder to xod.bkp and try running XOD again. If it works now, you can delete the xod.bkp folder; it is a broken copy of the built-in libraries.