I think we have a timeline problem. Doh.
I think FGma's confusion may stem from being herself misled.
Wm sounds like a precious, busy little boy, tangled up in some hard stuff, confusing for a little guy. His mummy and daddy and now new people. And visits and adjustments and expectations and pressure and acclimations and reacclimations. Natural for him to have some behavioral issues!
Which may have been on display from sun up that day!
Observation: foster parents may have split up that night to do a one-(foster)parent per child thing or..... maybe a little marital tension, spillover?
That morning, there was a lot going on, seems like.
If FGma is to be believed, FFCG left quite early....
Then there were these occasions of thrown dice and a tug-of-war toy (as in two children both wanted it). If Wm had a foster parent he navigated toward and that foster parent wasn't there and he had a foster parent who he preferred to bite and kick (maybe his only way to voice frustration or fear) and that foster parent was suddenly unhappy about the throwing or the tugging, what if Wm intended to run free and that foster parent essentially gave chase? If Wm took the tight turn, continuing on the veranda as it increased in height above grown, he may have tried to get away, going over the rail, unaware of the height differential, and that final roar may not have been a roar at all, but a shriek... death shriek. Fatal fall.
A marriage on edge, another fostered child in the mix, jeopardy, guilt, perfectionism -- any number of reasons a person might panic and opt for a coverup over a call for help.
Based on FGma's walk-through and recollection of events, it sounds like there was a time that FGma couldn't find either Wm or the FFCG. I think that may have been when FFCG was gone, driving the car...
I think there may have been some intentional timeline confusion -- maybe tea being made twice, or a sumersault on the timing of the roar. FGma almost makes it sound like FFCG was playing the mommy monster game in the side yard, out of sight.
Consider:
If Wm had a terrible, terrible fall.... fatal... and was swept up, carried to the boot of the FGma's car, a quick-thinking, cold or panicked FFCG might've returned calmly to the veranda, situated the FGma and the other child on the deck, with tea and crayons, then said, "Wm is too quiet" and went to "check on him" and the she might've carried on the mommy monster game as if she and Wm were playing. Out of sight of FGma, but unbeknownst within earshot of the neighbors who heard "children playing" -- laughing and roaring? Before driving off quickly...
And it could've been during her absence that FGma realized a second expanse of quiet... so she went to look for them (Wm and FFCG) down where they'd been playing.
I think all the elements are there, in FGma's recollection. Even she seemed perplexed with where they'd gone, how and when FMCG knew Wm was missing, where FFCG was when she went looking for her...
I think Wm was missing for as long as a half hour before FGma knew he was missing.... and that's the key to everything.
JMO