Hi
My children are very nearly 10 and very nearly 12, ie year 5 and 7. The younger one is supposed to be in bed at 8.30, lights out at 9 when there is school the next day, with the older one having the same arrangement an hour later. On Friday and Saturday nights they are allowed to stay up a bit later. There are exceptions - the younger one was allowed to stay up late last week to watch the Chelsea Man U match (and went to bed broken hearted!!) , although I'm not sure I'd have agreed to that had I realised how long the match would last for.
However, when the nights are light it's quite difficult for them to sleep. Plus I love the fact that they are outside all the time, not playing video games. So I do get a bit lax. However, the younger one is always in bed by 9 when there is school the next day. My older one seems to need a lot less sleep, but even so I insist he turns his light out by 10 as he needs to concentrate on school. I think you need to think about the needs of the chidlren, but if they seem to be tired in the morning then they need to go to sleeop earlier in the evening, it's as simple as that.