The director thought: "I'm going to start them out on silent films and so they'll think they're cool, and they won't know that they're really old. And then I'm going to do this. And then I'm going to do that. And it'll be fun. They're little blobs that have no individuality until they have start to have life experience.
"Basically," Bird says, "everything I was thinking was wrong."
From the very beginning of their lives, Bird learned, "they're different and individuals".
Pixar filmmakers have long tapped specific personal experiences as parents. Andrew Stanton, for instance, told The Post that while making Finding Nemo, with two small boys and a sprightly father of his own, he was very aware of being "both the son and the father at the same time". And Pete Doctor told The Post how a shift in his daughter's mood one morning, at age 11, helped spark the story engine that led to Inside Out.
Bird, for his part, recalls one moment with his eldest son, Michael, then a primary grade-schooler.
"There was a time when he just wasn't minding (my instruction) at all, and I almost wanted to spank him," says Bird, emphasising that he does not spank his children. "And then I looked at him and I realised that he was acting up because I hadn't been around in a while. And I just hesitated and I looked at him and I said: 'I know I haven't been around a lot.'"
Bird apologised. His son melted. They hugged. It was, the director says, "a great bonding moment".
"I put him to bed. And everything was fine. And I thought: OK, that's taken care of," he says.
"But it's not.
"The next morning is a brand new day and ... you're never done with parenting, really."
Such is life as a father, when you're Bob Parr - or Brad Bird.