Sonny Bill gives World Cup medal away
Williams said the boy, later identified as Charlie Lines, was "smoked" – thrown to the ground – by a security official after running on to the field following the All Blacks' 34-17 triumph over Australia.
Williams said he felt sorry for the youngster.
"I was walking around doing a lap of honour with the boys and a young fella came running out and he got smoked by the security guard, like full on tackled him. I felt sorry for the little fella.
"He would have been eight or seven, I kind of felt sorry for him.
"Hopefully [the medal] made it a night to remember for him," the All Blacks centre said. "It's better that he has it rather than having it hang up at home."
Williams came on as a half-time replacement for Conrad Smith and had an immediate impact.
He set up Ma'a Nonu for a try to push the All Blacks to a 21-3 lead.
"I knew that I'd been playing some really good football this tournament," Williams said.
"I knew that I was going to be getting some game time so when I got on there I just tried to bank on my preparation and just try and add something of value. Thank goodness it came off."
Australia pulled it back to 21-17 but Williams said this was the time to dig in and force a victory.
"I just don't want to lose. I just try and stay in the moment and do that job as best I can. In sport, you're going to have a winner and a loser and thank goodness we were winners.
"At 21-17 the bounce of the ball went our way and we scored a couple more tries.
"It's a very happy changing room because it's like a brotherhood and we all do it for each other," he added.
Captain Richie McCaw, flyhalf Dan Carter and centre Nonu are among the All Blacks for whom the World Cup final was their last appearance for New Zealand.
"We've got some special men leaving us but it just shows how good our roster is, with the depth of players that we have in our squad – all of us contribute," Williams said.
"It wasn't spoken too much about in the week but we knew we had to see those guys out winners. It was just a special night, not just for those guys but for everyone because we all pulled our weight."
Agence France-Presse
