Sandra,
I agree with the Miralax or suppository to get things moving for now. If she's been holding it in she may need help (from both ends) to move things along. There is a very gentle glycerin "bulb" made just for babies and toddlers that is premeasured with the solution inside. Our ped recommended this "as needed" because instead of a "hard" suppository, it's soft, wet and lubes the rectum to help any compacted poop come out gently. She explained that "hurting poop" is the most common reason for this "holding in" phase.
Make sure your daughter is getting enough fiber - everyday and you should be able to forgo the Miralax. I don't know about you, but I'm not big on meds, especially for the kids. I'd go with the natural methods for the long run.
Fruit, granola, fiber cereals/bars and watch out for bananas, cheeses and too much dairy. Kids will often choose foods that are fun/sugary so make these "snacks" fun too. Apples with peanut butter on them. Let her pick out some granola bars or fiber cereal at the store. Then let her sprinkle granola/cereal on top of yogurt. Also, make sure she's drinking enough water. Kids can get very dehydrated - esp with a laxative and in the summertime. Pick out a little "water bottle" for her that she can drink out of whenever she wants. Make sure you always have it in the car, on errands, etc. Not milk. Not juice. Just a bottle that you can freeze some water in overnight and fill with some water for her to have all day. If you make it a BIG DEAL, then she'll use it. Even better, let her pick one out.
Best of luck.