My DGKs are 9 and 7. They have been sewing on my machines since they were each 3 years old. One of the first things I taught them was to listen to Grandma... take your foot off the presser foot when I say 'stop' and go when I say 'go'. At that age they were on my lap or between my legs in front of me so I could stop any hazards. We proceeded from there to lift the presser foot, lower the presser foot, hold on to the threads as you begin stitching, pivoting, etc. By focusing on the 'listening' aspect I am confident that it is why, to this date, no fingers have been sewn on.
I hope this helps. Of course, I should add that at 14, I sewed on my index finger and to this day I can still feel the bounce of the needle on my nail.