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

I learned from my Dad who was a farmer and by necessity did most of his own mechanic and carpenter work. He was also a good welder and fabricator and built and modified farm equipment. So I worked along side him and learned a lot. That spurred me to get my Mechanical Engineering degree and work in mining equipment erection and maintenance. My daughter is my only child and she didn't like working on cars but she is pretty handy on house repairs and furniture refinishing. I'm trying to teach my grandkids about cars now. My granddaughter when she was 2 grabbed a hammer and was going to help Papa on the Duster. Luckily I caught her before she helped. Now that she's 7 she does help some.