Most 8 year olds should be in bed between 7:00 PM and 8:00 PM on school nights.
If it's a weekend then the 8 year old might stay up till 10:00 PM at the latest when they do not have to get up for school.
But if the 8 year old has school in the morning then they need to get enough rest to be alert in school so be sure the child is in bed at least by 8:00 PM.
You should keep a good sleep schedule and bed time for your kids so that there bodies are adjusted to the time they go to sleep.
Going to sleep at the same time every night helps the body to know when it's time to go to sleep so they won't have as much trouble sleeping.
An 8 year old child needs around 8 to 10 hours of sleep to be alert the next morning and if the child is falling asleep in school or during the day at home then it's likely the 8 year old has not gotten enough sleep.