Since many have said that this area was not ideal for a predator (the two-legged kind), my thoughts are that this was a crime of opportunity. Someone noticed these children and watched them, waiting for an opportunity. The fact that this person was nearby enough to take advantage of the situation when she went inside, is very creepy. They may have been watching in the bush. The fact that they took a chance in broad daylight, with grandma and possibly mum in the vicinity, tells me that they weren't afraid of being spotted. It was someone familiar to grandma, who would be able to explain away why they were there and why they had William. This has to be a neighbor or someone who was visiting a neighbor that day. How many houses on the street?