When I was a grommet, the kids on the block would get a crappy baseball card and one of mom's clothes pins and clip that card on their bike frame so that the spokes would slap it around and you'd get a sweet BRRRRRRRRR motorcycle noise. Apparently, today's kids are too lazy, baseball players have Myspace pages instead of cards, and moms don't use clothespins any more because someone's selling the Spokester - bicycle spoke noise maker.
The makers of the thing claim it is a safety device because drivers will hear your kid better. Spokester doesn't necessarily promote bicycle helmet use though. If your kid has a Huffy and has already installed all the aftermarket accessories available (streamers, license plates, baskets, bell/horn), teach her how to make a proper spoke noise maker and she'll be the envy of the neighborhood. Or, buy one for your $3000 road bike to make it sound like a $5000 road bike with fancy new Zipp hubs.
Photo from http://www.spokester.com