I have trouble with my 5 year old son that pulls a chair close to the drawer I have my film cameras on and presses all buttons he can reach. I have the habit to leave some of the with the shutter cocked (yes, those FSU ones that you need to select a shutter speed after cocking the shutter).
When I develop the films, I get plenty of shots with his off focus face. I print them all out, and I will hand them over to him once he's old enough, so they might mean something special to him.
I don't yell at him as he seems to love them and I would like eventually photography becoming his passion too.