Go for it. Entrepreneurship should be encouraged.Now they want to buy the full game and make a Youtube channel called "Kids play Kinect" and have people watch the LOL worthy results.
You might want to consider making them wash dishes, mow the lawn (in november...?), vacuum the place etc to pay for the game. You could make a youtube channel called "kids complain about doing household work" and have people watch the LOL worthy results.