Every person needs a different amount of sleep. If you don't want her napping, try having her go to bed earlier. Her need to sleep after school depends on several things. Does she have a long bus ride home? Does she have p.e. at the end of the day? Does she sleep well at night? Is she on medications that make her sleepy during the day? Look at these and any other factors before you get too worried.
it isn't so much a bad thing as a bad habit. At seven they're still used to nap times and stuff so it's normal to have her wanting to sleep, but she should get in the habit of of fighting sleep, next thing you know she'll be doing it in class or on the bus.
Not really... When I was 7 I would stay up till midnight and wake up and 5 and be hyper.. Now at age 14 I go to bed a 10 and wake up at 9.. It just depends on your sleeping habits. Every person is different. :)