is it a good idea to fix your kids' cars for them?

I prefer to fix things WITH my son, who right now is only 9! He loves to be out in the garage with me, so I like him to help change the oil, clean and disassemble parts and the like. We've had the mini bike up in the air for some maintenance too, which to me is a great way to get him going on single cylinder stuff, the foundation of all things mechanical IMO!!

He even thinks cardboard makes a suitable bed!!.... that's my boy!!