I don't know about you, but my kids sleep long hours on the weekends. I, then, have to nag them to go to bed early enough on week days since I know they have to get up early for school. My little guy likes 10 hours of sleep each night and still doesn't hesitate to go to bed by 8 or 8:30. He has never missed a day of school for being sick and I actually can't remember a day within the last year that he actually was sick.
All my kids are on Juice Plus, fruits & veggies in capsule form, which I think helps keep their immune system up. Juice Plus info
However, kids as well as adults, need to get enough sleep to keep their bodies strong. How much sleep do you think a kid needs?