MiraLax!! My daughter had/has the same problem. We tried all of the normal remedies (apple juice, prune juice, high-fiber foods, etc.), but nothing seemed to work for her. When she was three years old we took her to a pediatric gastroenterologist to see if there was something that could be done about her horrible constipation. She was potty training at the time and was afraid to poop on the potty. She would hold it in for days which resulted in extreme constipation. The Specialist recommended daily doses of MiraLax (one capfull in the morning and one capfull in the evening).
She is almost five years old now (and fully potty trained), but we still have her on a daily regimine of MiraLax (same dosage) to keep her stools soft. She is on a schedule of pooping every day, but if she misses a dose, she tends to get off her "poop schedule" and gets constipated again, so it is very important that she has it daily. If she spends the night with her grandparents or aunt, I pack a small container of MiraLax for them to use at their home.
She still has bouts with constipation about once per month (which get so bad I have to pick her up from daycare because she will just sit in the bathroom and cry all day). We took her to the Specialist again this past week in preparation for kindergarten and to make sure we should still continue with our routine. The Dr. said to keep giving her MiraLax since it appears she still needs it. We don't know if this is something she will outgrow or if this will be something she has to deal with the rest of her life. She is still too young to know anything definite.
MiraLax has been a lifesaver for us and I know it will work for you. It is a white powder that can be mixed into any drink. It dissolves completely and has no taste. It is safe for all ages, it is not addictive and you cannot overdose on it (per the Specialist.) It is not a stimulant, so it will not cause tummy aches or cramping. It just causes the body to hold more water in the stool making it softer and easier to poop. Once you start giving it to her, it will take two to three days for results. Just be consistent with the doses and she will feel much better.
If you want to talk or have any question, please feel free to e-mail me. I know how frustrating this can be.
A.