You do not want to hear about William Brown. He is irrelevant. He means nothing to you, just as he means nothing to Tom the moment evacuation becomes a certainty. Because William Brown is eleven, and Tom Riddle is fourteen, and eleven is beneath his notice.
He does not care that there is a dog, past the momentary 'oh, I hope it's housetrained', nor that there is a young woman (women are all the same) or that William appears to want to 'show him' places, because Tom Riddle is intelligent, and this boy is not, and that's all there is to it.
And he unpacks his suitcase in the guest room, and smiles stiffly at the cook who comes to check on him, and puts up with William Brown, who grins suavely and swaggers in to make himself known. He does not want a playmate.
William is disappointed when Tom tells him he's tired, but not much more than one would expect him to be. Tom makes up for it (seems to) at dinner; charming the mother, and making the father respect him, and the girl think him sweet, and the older son take him for a kindred spirit. Making William Brown apparently more determined than ever.
He takes it in his stride. The child is a 'William' and a 'Brown'. How much more average could he be?
Albus Dumbledore meets a subdued, more snappish, irritable Tom Riddle at the door of Mr. and Mrs. Brown's house, which is surprising, and a little bit annoying, as he had actually come to speak to William.
"What?" asks Tom, gritted teeth… and then looks up. He falters. "S-s-sir?"
"I…I…I… Take me away. Take me back to… I… I… Please…"
A voice comes floating innocently up from behind the garden hedge.
"Hypothetically speakin'," it says. "I mean, hypothetically speakin', if wizards were really real – I mean, really – there wouldn't be a difference, would there? Jus' supposin' muggles were all stupid. Jus' supposin'…"
Wizards, it is said, haven't an ounce of logic. Unfortunately for Tom Riddle, this is not so for William Brown.