Everyone needs to brush for two minutes -- a minute each for the top and bottom teeth. In Kid World, a minute is equivalent to about 10 seconds, which makes children less than effective at cleaning their choppers. Of course, parents of younger kids have to help anyway, so they can ensure that the time requirements are being met.
Older kids, however, may benefit from a little extra gadgetry. Pick up a kitchen timer, a stopwatch or anything else that will do the job. Set your chosen device for one minute while your child brushes the top of his teeth, then set it again while he scrubs the bottom. If you're looking for something a little more high-tech, there are toothbrushes available that play a popular song or flash bright colors for two minutes or so.