My daughter has suffered from constipation her entire life (she's 10 this month), and I have also my entire life (I am 42). My daughter also has an overactive bladder and has seen an urology specialist and a colon specialist. My daughter had the soiling issue as well, was also on Miralax as well as additional fiber. Constipation does cause soiling, and even if your daughter goes regularly, she can still have soiling. My daughter does. AND, she can still be constipated. My daughter usually is.
As another poster stated, it all comes down to diet at this point. Cow dairy is very, very hard on the body and the intestines. My husband and I have a casein allergy, so there is no cow dairy in our house. We use raw goat cheese and raw sheep cheese, occasionally we buy raw goat milk, or we use rice milk. We both suffer from a severe soy intolerance, so there is no soy milk or soy products either. In fact, the less you can use cow milk products and drink water, the better off your body will be in the long run. Calcium can be obtained through vegetables and other foods. Our family is also gluten intolerant, so we eat other grains and fiber besides wheat, rye, and barley (we can eat oats if they're not cross-contaminated).
Making sure we don't eat foods that we are allergic to or have an intolerance for has totally influenced how our bodies work, for the BETTER, and we are getting rid of many disorders and chronic health problems we once had.
Cow dairy is one of the hardest things on the human body. Since I've quit all cow dairy and started eating lots and lots of vegetables, my constipation has pretty much disappeared. It's harder to get my daughter to eat her veges. I make sure she takes an alpha-lipolic acid supplement, and I put pure organic premium ground flaxseed into as many things as I possibly can; it's a natural laxative. We also notice that when my daughter can't pee, it usually means she needs a BM and is all blocked up. Sure enough, she goes BM, then she can pee (without dribbling or having wet spots in her underwear) and all the pain in her tummy goes away. However, getting her to eat enough fiber and laxative friendly foods isn't an easy task (she LOVES carbs). Nor is it easy getting her to drink enough liquids. THAT is another big contributor to making sure she goes BM every day and doesn't get blocked up. It's a daily struggle in our household to make sure she drinks plenty of WATER.
Basically, you need to modify your daughter's diet to ensure she's eating as organic as possible with little or no prepared or processed foods. Her body is already having a hard time digesting and doesn't need to spend it on chemicals and pesticides and food colorings and dyes and all the other additives that the food companies put into mostly "dead" foods. We have noticed a very profound effect--for the good--on our family when we switched to an organic lifestyle two years ago.
I like Ericka L.'s post. Good info, Ericka.
Good luck.