My daughter is 8 years old and has suffered from constipation and the discomfort associated with it for many years. Despite a good diet like your daughter has, her digestive system is slow to process things. And, you wouldn't think she was having a problem because she has pooped nearly every day of her life, but the problem is, her poops are hard and she doesn't eliminate everything when she poops, so she gets backed up. The first few years were difficult because she would only tell me that her tummy hurt. After a lot of tests and dietary changes, I finally found an excellent pediatric gastroenterologist who understood and treated her condition correctly. Eventually, she had to have a "clean out" done at the hospital because she was so severely backed up, but it did help her. And she also had a colonoscopy, an upper GI, an acid reflux test and a number of biopsies done, just to eliminate other possible conditions. Thankfully, there weren't any. So, yes, we have had experience with constipation and it's a daily challenge.
Your daughter's diet sounds very good. I wouldn't eliminate dairy from her diet, but I would try to increase her fruits/vegetables to 5-6 daily servings. Also, serve brown rice instead of white, whole grain breads only, oatmeal, apples with the peel, and lots of water and natural fruit nectar. Limit the white sugar and white flour.
My daughter has also been on Miralax daily for the last 3 years. I was told by the nurse that they wean their patients off of Miralax 1/4 teaspoon at a time. Miralax has been great, but I would get a doctor's opinion on the dosage required for your daughter. I would keep her on the EXACT dosage prescribed until the doctor says it is OK to take her off of it. It may be years, but it's important to be consistent with Miralax.
Dr. Scott Sirlin is an excellent pediatric gastroenterologist and I would highly recommend him. He recently opened up his own practice in Loudoun County. I think he's in Ashburn, VA. Unfortunately, he no longer takes my insurance so I had to go to Children's Hospital, but I've been quite pleased with Dr. John Snyder too. Thankfully my daughter is doing much better and we've only had to visit Children's Hosp once or twice.
I sure hope your daughter feels better soon. Hang in there. You're doing well by establishing these good eating habits early in her life. She will appreciate your efforts when she gets older and eating right is just a daily habit for her! It's a lot of work now, but it is worth it!