They are close to child-proof. Kids can't hurt them. New sewing students (I teach 4-H) need to know nothing other than a straight stitch and back tack. These vintage machines can do that. When we use the newer machines, the kids fiddle with buttons and knobs, and the rest of the class is spent trouble shooting sewing machine problems. For the newer machines, (Thrift store and donation machines) I made a card for each machine with all the settings necessary for that particular machine. The kids still messed things up. About the only thing the kids can do to the older machines is adjust the feed dog tension. In the first class they learn safety, machine care, including oiling. They see the working parts of the machine, and have a greater respect for the technology. Perhaps they treat the machines nicely because they are old.
Be careful - collecting vintage machines can become addictive. They are scattered all over my house. I have to remember which one has which color thread for quick mending without having to change thread. With the older machines, I sew through heavy layers of jeans (including the seams)and even leather. Also, even my old treadle will drop the feed dogs for free motion sewing.