I like the idea, but the colors are a little too diverse (maybe take away a few for more visual impact), and the crotch area of the muscle layer looks a bit odd. Good concept, though, it reminds me of a weird German children's toy design. Keep it up!
Cute design, but something about the boy's lungs being brown just bothers me. Its like he has cancer from smoking for 40 years or something. I don't know. It might just be me.
i'm a bit thrown off by there being a face on the muscle body on the back (it is facing forward), and then no face on the fully clothed kid (it is facing backward). the faces on the back should be consistent.