Roxanne:
I understand your frustration as my son at about 3 yrs started going through the very same thing. He is 36 now and all is fine. You are obviously very anxious about the situation and it is difficult not to be highly concerned. However, you and your husband's very anxious response around your daughter is likely making her very nervous also. This is a situation that needs attention but with less emotional response. Making her sit and sit becomes a punishment which of course makes going to the bathroom a punishment and that certainly is not what you are attempting to do. Now with the Christmas holidays here and she is home from school, this is a great time to begin to resolve the issue. She is home and you have more control over her daily schedule.
You have consulted a doctor. What was his advice to you? He must have given you some instruction or suggestions. Have you followed through consistently with his instructions? The problem with this issue is that when children hold their bowel movements, they stretch the large intestine out of its normal shape into an even larger capacity. Also what can happen is that the bowel movement can dry out so passing the bowel movemet can be very painful. That is likely why she is so resistant to want to attempt to "let go" of the bowel movement. Realizing when I went through this with my son, we were in the 70's but the doctor recommended that I buy Maltsupex which is stirred into a favorite drink that the child enjoys and is supposed to loosen up the bowel movement so that it can get passed more easily. Personally, I don't have a problem using Miralax also. I wouldn't give a full child dose as that could be too much on her system all at once. However, I would give her a half childs dose, every day through the Christmas vacation so that she can clean herself out and her bowel system can begin to return to a normal size. That is what your goal must be, to get the bowel to its normal size. My son would wake up at night in terrible bowel pain. I finally had an idea - I had some prescription cough syrup that had codeine in it that I would give to him. He could get back to sleep but also because the codeine relaxed him, he would also have a bowel movement in the morning that was HUGE. The point, getting a child to relax, even by meds was very helpful. I defer back to the opportunity of this particular time of year. As she is on vacation, this is your opportunity. A little Miralax everyday throughout the vacation would be perfect. Also, if your daughter asks for a snack or resists vegetables, I would be giving her boxes of raisins, buy grapes and offer them at every meal, also apples, dried fruit if she enjoys it. Dried pineapple is very tasty. I'd encourage lots of fruit and any vegetable or salad that she will eat, I'd offer every night for her. I'd hold off on white bread, sweet cereals and see if you can get her to try some complex grain cereals. Costco has its own brand of raisin bran that is really delicious that even a kid would love. The raisins are plentiful and plump and fresh, not dried up, shriveled rocks. I think you can beat this in no time once she can return to having comfortable bowel movements and I believe it can be done by trying the procedures above. But please also try to relax yourself and not turn this into a battle which I bet it has become.