Hi there Jakes mum,
We went through a very stressful time with DD and constipation. She had a bout of constipation and from then on was to scared to go to the toilet because it hurt. Before this she was perfectly potty trained. But once the pain started that was the end of it. We had to take her to the hospital I think about 2 or 3 times because she would get to a point where she couldn't hold it anymore and would try to poo but it would be to big and too compacted to come out and would get stuck on the way out. Very scary thing to see your poor darling with her little bottom stretched to the max. Anyway this made things worse cos she got even more scared. Ended up taking her to the doctor who said to put her on parachoc (liquid laxative suitable for kids) and that she would probably need to be on it for a few months. The whole point being that it would soften the poo and they would realise that its not going to hurt when the do it and eventually go by themselves. All in all it eventually worked. But we took her off the parachoc and used a natural alternative. If you put 1 teaspoon of brown sugar in their sippy cup it actually makes your kid poo. Works fantastic for DD she became really regular on it. So maybe that scroller will help you.