Well, rewind, I bet somebody suggested harmonica because they thought it would be a cheap, portable instrument and easy to play. But from what I remember from being a kid, a harmonica is not easy to play. What kids do is zip up the holes and then down. Then they blow in and out, going Whew WHEW whew WHEW whew WHEW. And then they put it down and forget about it.

No matter what kind it is, the holes are too small and too close together. However, Hohner makes a kid's harmonica (saw it on Amazon). Maybe a kid could actually play music on one of those.

Why not visit a toy store and see what they have? You can probably find something that you blow on that has keys the right size for young fingers.