You can usually tell when you've had enough sleep. You wake up easily, refreshed and clear-headed. Compare that with the days you're running on 5 or less hours! The difference is obvious.
The need for sleep is even greater in kids because of the development that takes place while they are sleeping. Their sleep requirements for optimal development change from birth through age 18. However, it's safe to say a great many school-aged kids do not meet their sleep needs.
I ran across an excellent infographic that shows children's sleep needs and consequences of accumulative sleep deprivation. It's worth your time to read it.