IMHO, dad needs a project car. 1) It will tie up the pit so he can’t drive the family car in. B) The worst he can do is break one that’s already broken. Better yet, make it a favorite car from his past, and the shop manual to go with it. If and when he fixes one, you can find a good home for it, and get him a new project.
See link for New Beetle service manuals.
Dads retirement energy needs to be redirected. I would suggest the kids find an “antique” or “collectible” Italian or French made car (which never were actually associated with the words antique or collectible). Dad can spend hours researching and fiddling and trying to figure out how to find a left hand threaded woodruff key. All the while the kids know that these cars never ran (or barely) and never will again. Dad will have the thrill of trying to do something most sane people have declared as impossible. And in the end, his focus will be away from the family car. I know this, my family owned four Renaults and I don’t think we had any of them running for more than a week at a time.