I'm a firm believer (well, OK I could be swayed by opposing evidence) in kids getting more sleep than you think they need.
Too many people let their kids go to bed late (perhaps so they can spend more time together on workdays?) and I think it has a very negative impact on the kids.
My kids do not all need the same amount of sleep and it's also sometimes hard to make them go to bed at different times. One may tend to wake early, while another wants to sleep later.
In general, my kids seemed to do well with this kind of approach:
age 2-3 sleeping solidly for 13 hours per night (perhaps only 11 if there's a long daytime nap)
age 4-5 reduced to about 12 hours per night
age 6-8 down to ~11 hours per night
9+ still working on it, but it could be that less might be OK (only my oldest in this category and she usually needs the most sleep).
I think it works well to let them sleep until they wake on their own, but you need to put them to bed before they actually appear to be tired. Usually by then they are actually overtired and can have trouble getting to sleep. Also, if you're involved in something and miss the early signs then they can be really over-tired. We found that putting our kids to bed early (before 7pm) made it easier for them to sleep and then they could actually sleep as long as they wanted.
Habits are most important in forming sleep schedules, so it can take a long time to change. I myself usually wake up at the same time every day, even when I clearly have not had *enough* sleep!
On another note, I remember reading about a recent study where they examined the prevailing ideas about kids who have breakfast doing better in schools. The results of the new study were that it had nothing to do with breakfast, but only that those who had enough sleep would usually wake up earlier and therefore have time for breakfast! It was purely the amount of sleep that was leading to this increased performance in school.
