Mez, there is nothing wrong with going and getting a check up just for peice of mind.
Diabeties is a possible cause of increase thirst, but there are other less common things too.
The main concern with drinking loads of water is that it dilutes all the bodies other electrolytes and that in turn can cause problems.
How much sugar and salt is in her diet? these too can also cause incresed thirst.
The other thing is how active is she? if she is running around constantly she will be gettng thirst coz of that, think about when you do heaps of exercise or are much busier than normal, your thirst increase
2.5L is a lot for a little girl i would have thought. although thinking about it MJ probably drinks a little less than that BM in a 24hr period

Hopefully its just that she is very active and enjoys her water