In a private moment hours before becoming Canada's 23rd prime minister, Justin Trudeau was climbing the stairs in an empty Parliament Building, holding his two eldest children by the hands, when he made an unscheduled stop in a hallway holding the portraits of former prime ministers.

"Who is that?" Trudeau asked his son and daughter.

"Grandpapa," Ella-Grace sings.

"That's Grandpapa Pierre," Trudeau told his children. "Today, he'll be thinking of us and we'll be thinking of him."

Prime Minister Justin Trudeau speaks to his daughter, Ella-Grace, in his office on Parliament Hill. (CBC)

He quickly raised a hand to caress the frame around his father's portrait before abruptly announcing, "OK. Let's go."

It would have been impossible for Pierre Trudeau not to have played some role in Wednesday's swearing-in spectacle.

Over the several hours Justin Trudeau spent in front of a camera and tethered to a microphone for a documentary by CBC's The National, it is clear memories of his father were threaded through his thoughts.

Pierre Trudeau wasn't just waiting for him in the hallway leading to the House of Commons. The memories of his dad were waiting for him in his new office. They had to be outside of 24 Sussex Drive when Trudeau pulled up in front of the empty executive mansion for the first time in years. What was Trudeau thinking as the armoured sedan wound around the drive of his old home?

"This is weird," was all he would offer.

Trudeau and Peter Mansbridge look at a portrait of his father, former prime minister Pierre Elliott Trudeau, on the wall of Centre Block on Parliament Hill.

Sharing extraordinary moments

Trudeau would later insist publicly that few of his thoughts on this day would be dedicated to the father who governed Canada from 1968 to 1984 except for a brief nine-month interregnum.

"My thoughts today — sorry Dad — aren't mostly on him," he told reporters outside Rideau Hall.

His relationship with his father rarely ventured into the political. Trudeau told CBC the one time he asked his father for political advice, the conversation was remarkable for its awkwardness. It was the last year of Pierre Trudeau's life and he was fading from the effects of prostate cancer.

"He said, 'What do you want to know specifically?' And I said, 'I don't know. You want to do something that's the right thing and banks and big business push back at you, and they don't want you to do it. How do you deal with those kinds of pressures?'"

Trudeau and his children Ella-Grace and Xavier look out over Ottawa from the top of the Peace Tower on Parliament Hill. Before he was sworn in as prime minister, Trudeau raised a new Canadian flag at the top of the tower. (CBC)

"He sort of gave me an answer. It was a really stilted conversation the way we never had stilted conversations.

"And then I sort of realized that everything that I need to know about being prime minister he had already taught me, about being a good person."

Pierre Trudeau also taught his children to savour some of the extraordinary moments that might seem routine to the children of a prime minister. Justin Trudeau began to share some of those moments with two of his children on Wednesday.

"Do you want to see Dad's new office?"

Peter Mansbridge visits inside the Prime Minister's Office with Justin Trudeau on his first day as prime minister 7:40

Of course they did. And he had stories for them about that office. This is where he cartwheeled with his younger brothers Sasha and Michel, he told them. The tour included gazing at family photos and Wilfrid Laurier's pen while pointing out the secret doorway that once obscured a hidden television set. But the office was much different when Pierre Trudeau worked there.

"Lighter walls, less dark when my dad was here," his son said.

Raising the Peace Tower flag

Shortly after the sun rose, Trudeau, clad in faded jeans and a sweater, hoisted his children into a space the size of a telephone booth atop the Peace Tower so they could bring down the massive Canadian flag that had flown there the previous day.

Peter Mansbridge accompanies Trudeau and his kids as they visit the Peace Tower 1:32

Once that Maple Leaf was brought in, Trudeau clambered into the narrow space and hoisted the flag that would fly on the day he would become prime minister. He hopes that flag will be returned to his children some day.

It was part of a day that he carefully choreographed, right down to the choice of song the pipers would be playing as he ambled up the driveway of Rideau Hall with his new cabinet (The Bonnie Banks o' Loch Lomond).

But there would be one difference from his father's path that he would insist upon.

A black-and-white photo of his father surrounded by the sober-suited men in his cabinet as they strolled toward Rideau Hall had found its way onto websites and front pages everywhere earlier this week.

There would be no repeat of that image on this day, the younger Trudeau said.

"A bunch of older white guys in morning suits."