We've been playing Just Dance and Wii Fit quite a bit lately and I was thinking a small wrist-band or watch type controller would be handy. The wiimote gets heavy for my 5-year-old and you really just need the motion sensor for these games and maybe "A" and "B" buttons to advance through menus. Does this exist already?
There is no such controller for Wii or Wii-U currently (but nobody knows what Nintendo is cooking up)
This is pretty much a technology/cost barrier. as the motion detection hardware is quite large (although it has been getting smaller if you notice wii-motes now come with motion+ and are the same size as original wii-motes). Hardware that small that stil works effectively in a gaming environment is also probably quite expensive even if it does exist,
If you want to play motion games without the weight of a bulky controller, try investing in the Kinect. Now that the Xbox One is out you could probably find a cheap Xbox 360+Kinect bundle or something.