According to this article in the WSJ they need a lot. The article states that children ages 5 to 12 need between 10 and 11 hours of sleep a night. This guideline comes from the National Sleep Foundation, a nonprofit sleep education group. The article can be summed up with one finding...on average our nation's kids are not getting enough sleep.
As parents we have learned to try and put our children to bed as early as possible because a) it takes them a while to get to sleep and b) no matter how late they go to sleep, they still wake up at the same early time, especially on weekends.