I usually let my 8 yo young fella into my garage for the first 10 minutes of my workouts. I've got a punching bag that he loves smashing the crap out of, and a 3kg dumbbell that he likes to do 1 arm oh presses with, around 30-40 reps. He also does hilarious negative single reps on the dip attachment (basically just collapses once he gets up on it).
But when I actually start the workout I piss him off back to the house. Gyms for kids are just an accident waiting to happen. Just let them run around outside, kick balls, climb trees etc.