As we were studying the bones and learning how muscles are attached to the bones using tendons, the lad looked at his hands and feet and said "mom, are those my muscles that stick out when I move my fingers and my toes or they tendons?" I told him the lines you see are your tendons...but they help your muscles to work. He was FASCINATED.
To that end we found this video to help us understand how the muscles of the hand work. It was a bit longer then what we wanted but gave us a ton of information.
To help us further understand how muscles work we thought we'd put together a robotic hand. We found one on Mystery Science that looked doable. We liked how it showed the finger curl but disliked how it worked/didn't work as a whole unit.