My son began having headaches around 3.5 or 4yrs old. Headaches to the point of throwing up :O(
I began to learn that this is a part of his life when he plays too hard. So, I tried to "prevent it" by having him lay on the couch after preschool with a warm washcloth and have the Tylenol ready. I still have the same "rule" after school......rest for 20 minutes, eat a snack, then homework
My son is now almost 13 and still gets these migraine-like headaches, on occasion. He is very well hydrated, so it's not that, I believe it's just too much activity...... he is ACTIVE! But because he's so active, I make him go to bed at the same time as his 6yr old brother which is 8:15, and he gladly does it.
If your daughter is going to be "prone" to headaches like my son is, then you will begin to pick up on the "signs" and be able to read her face better to associate the way she feels. That came naturally for me because of the headaches. Sometimes, my son didn't even tell me he had a headache, and I gave him Motrin because I knew he had one. And I was right, he had one, he "just forgot to tell me".
Kids at this age become so independent and play SO HARD! I'm sure your daughter will be just fine :O) Just NEVER run out of Tylenol....it's the worst when you do and you need it for one of these headaches.