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

Depends on the repair and the mechanic learning. My daughter is a tiny thing @ 5'4" 90lbs soaking wet (while her 11 year old brother is 5'6" 125.)
She'll do a lot by herself, but sometimes Dad just gotta get in there and do some of the tuff bull work she can't do. Straightening out the front end after the fender bender was my job.