I agree with S.M. here that a curfew is a guideline and should be based on your child's maturity as well as the cultural norms where you live. For what it's worth, my parents are ultra-conservative and my weeknight curfew was negotiable depending on the reason I needed to be out on a weeknight and weekends were 10:00 at that age, again negotiable depending on the reason. On a date with the HS sweetheart? 10:00 (which really meant in the house by 11 and usually in the driveway by 9:45)
She will be attending a carnival in the summer and probably wants to be there when it gets dark. Let's be honest, carnivals are way cooler in the dark!
Just my thoughts here...
1. Is she responsible enough to be out later?
2. Does she generally respect your guidelines and curfew?
3. Does she have to get up for school the next day?
4. Does she have a cell phone or could she take yours?
I don't know exactly what I would do in your situation, but at that age my parents would have let me stay out later with the understanding that I would be with a group and call periodically to check-in AND would be in by 11:00, not one second later.
To me, this is a special occassion and something fun for her to experience with her friends. You know where you live and you know what is considered "safe" and what isn't, but I would probably let her stay out later and see if she can handle the responsibility. If she can't, well- back to 9:00 and no flexibility until she can handle it!