The Littmann Lightweight II S.E. Stethoscope by 3M was recommended by our medical professionals.  Not exactly a toy, you'd think (and rightly!) -- yet our 4 young boys love it, too.  They check out their own and each other's heartbeats, as well as enjoy all the stomach noises!  The adults use it mostly for the ordinary lung and blood pressure checks that make it a standard household item.  Importantly, this particular Littmann 'scope compares well against far more expensive equipment and is thus the one I most highly recommend.