Note: I shamelessly stole this idea from High Five by Highlights!
Cardboard tube (paper towel size)
Have your child wrap the cardboard tube in aluminum foil and then decorate with stickers. Have your child hold it up to your chest and she can hear your heartbeat! Run around and then have her listen again and talk about the difference between your resting heartbeat and your heartbeat while exercising!
NOTE: Small children may not understand what they're hearing...my daughter says she couldn't hear my heartbeat, but I could hear hers just fine and she has MUCH better hearing that I do....they've been told it sounds like thump-thump and that isn't always the case!