Those aren't my words, it's what I have been reading in the news today. The sources say that the Wii Fit and balance board do not help children to get fit. That's true, but in conjunction with other things it can help. The Wii Fit is never going to replace exercise completely, especially for children.
Kids should be getting plenty of exercise naturally. They should be playing outside in their gardens, with their friends in the park or out on the road. If it's too cold (or unsafe) they can play games indoors like Hide & Seek, or dance around to music.. that in addition to playing 'active' games on the Wii.
The Wii Fit is primarily for adults. It helps motivate and get us moving when the last thing we want to do is travel to the gym or undertake strenuous exercise. It spurs us on and in doing so we make changes to our diets, and start upping the ante a little by adding more activity to our daily life. That's what it did for me anyway.
It's a great tool for adults and there are some fun games for children, but if that is the only source of activity in your child's life then it is safe to say they are not getting the exercise, that as a growing human, they need.