They are some groups trying to validate balance assessment tools for children and adolescents.
If you have a force platform available, you could measure the Center of pressure (COP) displacements in different conditions (bilateral stance with eyes open, bilateral stance with eyes closed, unilateral stance on dominant and non-dominant leg). A cheaper option could be assess the balance using a Nintendo Wii board.
An easier and cheaper option might also be one leg stand (DV: time until using the second leg). With the above mentioned pedalo test you may measure any correcting movements as well.