Did some shooting from the new bench with the .17 this evening. My son went with me (going on 7 y/o).
As a typical young boy his version of sitting still means staying inside a 5 foot radius. His usual fidgeting around beside me had no noticeable effect looking through a 12X scope. Might have noticed minor movement if I was really going for groups with a target rifle. Only time I had to ask him to settle down was when he started beating on the table top making it vibrate, lol.
I consider this hugely successful as he was sitting on the same seat as I was.