When they explained the augmented reality game, it got McDonald thinking about how something similar could be translated into a banking environment.
"All of a sudden you're going: actually, that's really smart."
The connection between McDonald and the ASB's youngest staff didn't happen by chance.
More than a year ago, a visiting US speaker explained how companies were using school children to "reverse mentor" in areas such as technology and social media, to provide fresh insights.
"We always ask ourselves the question whether we're using all of the resources available to us," says McDonald.
"If I look back, one thing I've learnt very early on in my years was you don't solve the problem yourself.
"You've got lots of resources there that you can use."
ASB has now developed its own reverse mentoring scheme between the bank's executive team and two dozen staff born after 1982 - the year of the Falklands War, the Commodore 64's release and Olivia Newton-John claiming number one on the music charts with Physical.
The group is dubbed Gen YLT - a play on the ELT acronym for the executive leadership team - and each executive, including bank boss Barbara Chapman, meets a couple of millennials each month to provide a different perspective and fresh ideas.
Alice Selby, a regulatory compliance officer in the wealth and insurance division, who mentors Chapman alongside retail implementation manager Michael Evans, says the mentoring is a two-way flow, with both sides offering insights.
"All the mentoring relationships are different," she says. "Some go for a wine, some go for a coffee. So far we've always met in Barbara's office, but we're working on that."
Traditionally, mentoring involves someone senior talking about how you can develop your career and work through the issues, says Evans. "In a reverse mentoring relationship it's us talking to a senior leader about the challenges they're facing in their business and what our perspective is on it, because our perspective might be a little bit different to the perspectives they're used to hearing."
One thing I've learnt very early on in my years was you don't solve the problem yourself.
Selby says there are plenty of buzzwords associated with millennials - some positive, others less so.
While millennials might be criticised for having no patience and wanting everything now, the flip side is that with the ever increasing pace of change, they are well placed to make things happen, she says.
"Digital is our native, it's second nature to us."
McDonald, who identifies as a baby boomer, says it's not that his generation weren't coming up with great ideas when they were younger, it's just that the pace of change has left him behind the curve on the tech front.
Getting the programme off the ground hasn't been without its interesting moments.
Given a clean piece of paper and use of the company boardroom, the founding millennial group promptly parroted back the company strategy to McDonald, who is one of two executives overseeing the programme.
Instead of providing a fresh perspective, the team had been discussing governance structures and administrative frameworks - and ensuring it was all carefully minuted.
What it does is continue to reinforce my view that we've got bags of talent within the organisation and if we can really find a way of leveraging that, that creates an enormous amount of value for us.
It made McDonald reflect on how quickly organisations institutionalise new staff and that the only way to combat that was for senior leaders to provide the environment - the "air cover" - for younger staff to try out new ideas.
"It doesn't matter what the initiative is, if it isn't owned by senior leaders within the organisation, if they don't show passion around it, it dies."
Gemma Macaulay, a senior product manager within retail and business banking, who reverse mentors CFO Jon Raby, says the group developed four key principles: dare to try; be epic; make it happen; and rock the boat.
Reverse mentoring is the first of several projects to come out of the group, including networking events aimed at its 1300 millennial-age staff, using social networks to pull ideas from the wider group and pushing reverse mentoring further down the organisational layers.
So does McDonald feel he has changed as a leader?
"Well I know about Pokemon now," he says. McDonald says Chapman's challenge to lead the group "was just magic to me".
"I haven't changed.
"What it does is continue to reinforce my view that we've got bags of talent within the organisation and if we can really find a way of leveraging that, that creates an enormous amount of value for us."
