Wii Music was introduced by Nintendo this Summer at the E3 2008. I am totally not getting Wii Music, but that is because I am way too old. Nintendo's famous game designer Shigeru Miyamoto visited a Japanese Kindergarten and played Wii Music with a group of kids.
I'm not sure if these kids have been hand picked or if all 3-year-olds in Japan are already playing the Wii.
In any case Nintendo published a video of the Miyamoto Kindergarten Wii Music jam session. The video shows how well little kids like Wii Music. It is really just for them.
You can see the video on the Nintendo Site.

Wii Music is available on Amazon.com.