I think I'm going to start including a free whistle tamper with each pipe I sell, I've gotten some really great feedback from my mailing list and sold a couple as well.
Anyhow, plain old wooden whistles are really easy but I'll warn you to make a few because all of your daughter's friends are going to want them as well
Here is a great video on it, I cleaned things up a little, e.g. I turned the spindles and drilled on a metal lathe and cut the notch with my strip sander but the ones in the vid work just fine for the kids. I'll add you can cut a wedge off under the airway to make them fit in a small mouth easier or if your not using dowels like in the video just turn the spindles down smaller. Also don't stain them.