Children are still in the process of growing both physically and mentally and that extra sleep is what their bodies need to grow and mature in a healthy way. Once you've stopped growing, you don't need as much sleep, which explains why people tend to need less sleep as they get older.
Babies, children, and teens need significantly more sleep than adults to support their rapid mental and physical development. Most parents know that growing kids need good sleep, but many don't know just how many hours kids require, and what the impact can be of missing as little as 30 to 60 minutes of sleep time.
One of the reasons it's so hard to know when kids are getting insufficient sleep is that drowsy children don't necessarily slow down the way we do they wind up. In fact, sleepiness can look like symptoms of attention deficit-hyperactivity disorder (ADHD). Children often act as if they're not tired, resisting bedtime and becoming hyper or unstable as the evening goes on.
Getting too little sleep on a regular basis can have even more dire repercussions, especially when you're young. Too little sleep can lead to not growing like you should. Lack of sleep can make it easier to get sick.