There is witness testimony that children were heard playing (by PS) between 9 and 9:30.
So, unless he was lying, the FGM was lying, and the FM was lying (collaborative lying) William was there at that time.
The idea of other children having been in the area at the time were dispelled, I believe, due to it being a school day, 9pm being after school started for the day, and no other children being in that area at that time.
I personally am trying to make this 'new' scenario fit with using all of the known facts.