When my daughter (10years old) gets tired her pupils seem to enlarge. Is this a problem?

Hard to tell... Your pupils can dilate if you are angry, scared, or stressed -- it's part of the 'fight or flight' response. I'd advise having her seen by her doctor (including a thorough eye exam and vision screen) just to make sure there's no underlying issue.
Nope. A normal reaction which is mediated by the natural hormonal reaction to stress.
Not usually. If your daughter's eyes dilate when she is tired this may be a normal pattern or variant function for her. If she is not complaining of eyesight problems i wouldn't be too concerned but if she has never had a routine eye check she should be checked anyway to rule out other issues if possible.