Personally, if the kids show a desire to be there I see nothing wrong with it. As Streets said stress safety and the right way to do things, and keep the number of kids to point that you can actually keep an eye on them.

At the age of your kids the one thing I think I would do is make sure the garage is locked when your not there. If they think of the shop as a fun place to be, chances are they will be there when your not around. Unsupervised any shop can be a dangerous place.