Stew up some apples with a bit of brown sugar and get her to eat that - this really helps ds.
Another option is to stew up lots of dried apricots until they turn into a sort of fruit spread. We tend to mix this with his porridge in the morning, but if she is gf (I think you said she was at the moment on the other thread) you would need to buy some gluten free/pure oats to do that.
Both have a high sugar and water content, which is what you need to get her going.
Dark chocolate (decent 75% cocoa stuff) is also a good natural laxative.
That said, if its an impaction none of the above will be enough. She will most likley need stool softeners and some movicol, especially if its really painful.
What happens is that over time, with repeated episodes of constipation the bowel gets stretched and becomes less efficient at pushing out the waste. It then builds up and up in the bowel and fresh waste from further up the gut can leak around it, hence the soft stools. This can also present as watery diarrhoea. The problem is that the large amount of impacted waste in the bowel becomes very hard and extremely difficult and painful to pass, which can then lead to children avoiding going - viscious circle.
How long it takes to build up varies from child to child, but with my ds, it tends to be roughly five days inbetween.