My toddler sits on my shoulders while I do pushups, and we once caught him doing curls with my wifes 5lb dumbells. (it was so cute cause he could only do 1)  But he's like 3, so I let him  do curls with his stuffed monkeys when he wants to mimic me. Tonight at bed he was actually mad cause he wanted to "exermacize one last time before sleepies."