I'm returning to FM's story but making up my own theory of what happened.
What if FM thought WT was somewhere safe when he wasn't? They'd gone for a walk earlier, kids rode their bikes, played in the yard, took their shoes off and came into the house, had a bite to eat and seemed content to sit and draw pictures on the deck, FM took pics of WT and sister drawing, they're barefoot, FGM reading the local paper.
10.05 am- 10.10 am - FM and FGM went inside, chit chatting or deep in conversation about concerns for WT or other issues, maybe, she was looking around the house to check nothing else needs doing for this sale, FGM prepares another cup of tea, both still chit chatting and FM lost track of time.
Iirc, LT came inside to use the toilet in this time period, and FM said she didn't need assistance, so was she even aware LT had come inside to use the toilet, or, did she find out later after WT went missing and questioned LT about the events?
Before LT went to the toilet, WT had put his shoes on by himself and said to her, 'I can see daddy...I'm looking for daddy.....or, daddy's here?' (not sure exact words LT remembered)
Or, After LT returned from the toilet, WT had already shoes on and he says, 'I can see daddy.....' and he looks over railing and then, runs off into the yard.
FM hadn't seen WT put his shoes on by himself before. A lot of kids are difficult at this age about shoes, especially if the mum is in a hurry.
10.25am - 10.30 am - When FM returns to the patio with FGM and cups of tea, WT is gone and so are his shoes! She's shocked and then, goes into a panic!
10.25am- FF texts he'll be home in 5 minutes, FM desperately searching around the yard and part of the street.
10.30am - FF returns, FM tells him WT has disappeared and both search and make contact with neighbours to help.
FM is acting cagey about the shoes because she never saw him put them on. WT and LT had been left unattended for...15 -20 minutes? That's a long time for littlies to be left alone and imo, she feels guilty and helpless. She risks losing LT and is devastated she let William down.
In 15 - 20 minutes a lot could've happened. WT could've have run god knows where, there's no fences, he's not familiar with the area, he's not a 'wanderer' but there's always a first time. WT could've been abducted anywhere in the street, front of someone's yard or hit by a car further down the street and taken away......
In saying all that, FM is sensitive about questions regarding the 'shoes', the amount of 'time' WT and LT were left alone on patio, when LT 'went to the toilet', when she 'heard WT's growling'.
I'm guessing FM drove around searching at approximately 10.40 am and returned to make 000 call at the house at 10.56 am. Time driving 10-15 minutes at the most.
JMO