How do I train the kids to sleep earlier?

I stopped their afternoon nap and let them sleep around 8.30pm and by 9pm they should be sleeping. Initially it was quite tough for them to get adjusted but I gradually adjusted their nap time and let it be a routine and everynight now they will have routine and by 9pm they will be sleeping as one of the nurse told us that kids needs many hours of sleep to let their brain and body to repair their cells and they will feel much fresher and alert next morning for school.