Great points Frogwell.
Yes, it seems the MFC arrival home was something the kids always looked forward to so I can definitely imagine William waiting in the driveway for him, maybe he heard something and ventured around to see. Especially if, like you said, they parked in the lower driveway.
Okay, I've found a reference from the inquest to what you said about the children growing restless and then going to see if they could see the MFC's car.
“(FFC) told the court that William had been “getting very bored” just before he vanished and they were waiting for his foster father to return from the nearby town of Laurieton.
She had been expecting her husband to return from a visit to the chemist and a telephone conference meeting between 10.15am and 10.30am that morning.
“William’s sister was still drawing,” she said.
“He was getting very bored with the game. He was running up and down.
“I said to William (and his sister) ‘you know what? Why don’t we go down and see if Daddy’s car is here yet’.” (ref: William Tyrrell’s foster mum heard a 'high-pitched' scream while searching)
The FFC says “down" (as bolded by me above) which makes me think she’s referring to the lower driveway?
Also, she says this was "just before he vanished" which really does seem to give a legit reason as to why he may wander.