I have just been through a horror stretch of constipation with Mason.

Try and increase her clear fluids. If you only give her water then maybe try diluted apple juice or cordial so that she drinks more. Prune juice is good too if she likes it.
Offer lots of fruit like apples with the skin or stone fruit if you can get it. Bananas can make constipation worse.

If she is holding on because of fear then maybe try a gentle stool softener from the chemist. Mason was terrified and held on for a week and half which made it so much worse. By using a laxitive in the short term over a week or 2 but only a mild dose you can get the child retrained that poos don't hurt anymore.
I also am giving him some yakult as his constipation was more than likely caused by the antibiotics killing off his good bowel bacteria. I'm sure this has helped and he loves it.