This is just my jumbled speculation:
Adults are sleeping separately. To maximize parenting? A harbinger of marital tension?
Morning begins, busy. Wm is emptying cupboards. IMO he's still in his jams.
Still speculating -- the male caregiver leaves. To get a newspaper, perhaps to drop off a script. Meanwhile, the remaining individuals have breakfast. Wm dons his Spiderman tee and Spiderman duds.
Male caregiver returns home. Foster children race to the yard to meet him.
Foster children play on their bikes.
Female caregiver makes tea (the first of two times IMO). Everyone gathers on the veranda. Photo ops. Foster grandma has her tea and her newspaper, foster daughter is alternately playing with Wm and drawing. The photos are taken.
Male caregiver leaves again, for his virtual meeting.
Wm runs around the side of the house but turns sharply, onto the second veranda and not the yard. At the length, he is now a full story above ground, but it took him only a step or two to access the veranda. He may not realize that, while the veranda hadn't changed, the landscape below it had fallen off.
Possibly he is pursued, over correction from rambunctious dice rolling.
And Wm crashes into the garden.from a height. Death, immediate.
IMO I think, in that one moment, one person made one calculated decision -- that no one can ever know about this.
I think the neighbors were serenaded with a raucous exchange of Mommy Monster, only zero children were playing. And only one adult.
While the foster grandma sipped her tea second tea?) and read her paper at leisure and the foster daughter lost herself in coloring, I think the female caregiver drove off in the foster grandma's car.
If she were truly driving for the purposes of a search, IMO she'd have told the foster grandma to keep an eye on the foster daughter and call her cell if Wm shows up.
I think she did not. I think she left, in secret, fairly confident that the two on the deck were well occupied.
I think, however, that she was gone long enough that the foster grandma noticed, and went in search of....
I think the female caregiver arrived home barely before the male caregiver did. I think she must've pulled in just ahead of the foster grandma coming to look for her.
I think no one but the female caregiver knew about that drive -- except perhaps for neighbors who may have heard the car. And I think it would have remained a secret. If not for the trucker making eye contact.
I think the foster grandma was easily moulded to think mere minutes elapsed between Wm and the female caregiver's exodus from the deck --
When the male caregiver arrived, the female caregiver gave off the impression IMO and by design that Wm had JUST gone missing and she had JUST begun searching -- five minutes, could have been 20, he was just here and now he's gone...
The drive is key.
JMO