Valve might make some of the best games for grown-ups around, but that doesn't mean they're a one-trick pony. Speaking with CVG, Doug Lombardi has said that they're actually interested in making games for kids as well:
There's a lot of people at Valve who are parents and would love to make a game for kids.
We all play the Wii a lot and we think that the proper way for Valve to approach the Wii would be to make something cool designed specifically for it.
Not a concrete announcement by any stretch of the imagination, but Valve and/or Wii fans, don't let me stop you from dreaming the good dream.
Valve feels the Wii love [CVG]