PS has been developing memory problems--I'd want to know when he first said he heard the children playing that day. But also, and this could apply to both PS and JW, people remember what they expect to remember. When you know there were children out playing, it's easy to think you remember hearing them.I think that would require both JW and PS to be directionally challenged. As well as both of them being unable to tell the distance between them and the sounds of the children.
When there are two independent witnesses, it seems to make it more probable that they heard William and his sister playing. imo
However--rather than think everyone's lying, deaf, confused or deceived, I find it easier to suppose William fell down a drain, perhaps the one that was barely inspected because cobwebs appeared undisturbed.