An 8 year old should get between 8 to 12 hours of sleep and the more sleep the better but if the 8 year old is only getting 8 or 10 hours of sleep at night then it's okay as long as the 8 year old child is feeling fully rested.
If your 8 year old child is feeling groggy and tired during the day then it's a sign that the child is not getting enough sleep at night.
You should make sure your 8 year old gets into bed at night soon enough so that they have enough time to get to sleep and get a good nights rest for the next day.
It can take between 10 to 30 minutes before a child or anyone is fully asleep once they get into bed so you need to factor in that amount of time as well.
If the child is not rested and is tired during the day then the child won't be able to function properly in school and won't be as alert.
The 8 year old may fall asleep in school as well and if they are then the 8 year old is not getting enough sleep at night.
Sometimes your 8 year old may go to bed but not really be asleep so check on them after 30 minutes or so to be sure they are actually asleep and not just under the covers on the phone, tablet, computer etc.