When my Daughter was that age, she got constipated and would, on purpose, withhold her poop.
We... had to see a Pediatric Gastroenterologist.
HE said.... that once a child starts to withhold their poop.... it is then a 2 part problem: 1) medical 2) emotional based problem.
He said, everyday, he sees toddlers like this. They are toilet training. Or are forced to only poop on a toilet, when they are not ready. The child, then withholds their poop.
It then, causes internal problems/blockages/possible Encopresis/emotional problems... and it is just a vicious cycle.... because the more the child holds in their poop... the harder the poop becomes internally AND then when it does come out, it causes pain. Therefore, the child, will again, avoid pooping, because it hurts when coming out... it is hardened... and internally the more they hold it in, the more pain they get. Then the vicious cycle continues.
For many children, he said once Constipation sets in... it is very hard to undo... the process of pooping normally again.
For our Daughter, it took THREE months... for her to Psychologically feel comfortable with pooping again... AND for her stool to become a normal texture again. We had to give her... prescription remedies. Per the Doctor.
You said your Daughter is backed-up. Did they do an x-ray????
If the only way she will poop, is in a diaper... then LET her.
You did not say, if she is avoiding the toilet or only going in her diaper or just not going at all.....????????
IF constipation gets worse.... then "Encopresis" can occur. Look it up online. I am sure, you don't want this to occur.
IF she is not even pooping in her diaper... then... it is probably because, her poop cannot come out anymore. AND it probably hurts (internally and externally)... which is what happens with constipation. So then the child AVOIDS pooping.... more.
Try giving her pear juice. Whole unfiltered apple juice. Prune juice.
I think, it is time to take her to the Doctor....
If her bowels becomes SO backed-up... then it is a real MEDICAL problem... and her bowels are obstructed and bulging internally.... and that is not good.
Take her to the Doctor or ER....
The Pediatric Gastroenterologist we saw for my daughter... said NO suppositories... on children....