I feel so badly for that little girl and her family. I would be removing my child from the school and finding another one that has a more nurturing ethos whilst also going down with all guns blazing. It sounds as though the 'investigation' was a smoke screen to fob the parents off and was not transparent; the parents certainly do not feel informed by the outcome.
My eldest has chronic constipation at the same age and the first school he went to would not help him (if he did go to the toilet whilst he was there) and would often leave him in soiled clothes because of faecal leakage (despite me sending clothes, wipes etc and even telling them to call me as I would have gone in and sorted him out). After trying to 'work with the school' for sometime in the end I moved him to another school and, funnily enough, his constipation problems cleared up pretty much over night. I realise that this little girls problems may not resolve themselves but a) she should not have had to clear up (what about the other children, you can't expect a 5 year old to clear up bodily fluids thoroughly (bless her)), b) being left in wet clothes is just degrading and wrong and c) why was she then punished (I actually don't know many schools that still use the stand and face the wall punishment)?
I hope the staff and governors of that school reflect on their tag line "We are proud of our school"....