It is essential that children get enough sleep. Children need sleep in order to grow and develop normally and to concentrate on learning at school. There are many reasons why children may not get enough sleep and there is plenty of advice out there. If you have tried all the obvious things (ensuring a good diet, lots of exercise, a good bedtime routine with NO screens in the bedroom and all screens removed at least 30 minutes before bedtime) then you may wish to seek medical help. You should try your GP first, then if it is a severe on-going problem you could request an appointment with a paediatrician.
There is excellent advise on the NHS website, just google: NHS sleep problems in children