The pair, with two others, will take part in the challenge on Sunday alongside other Rotorua teams to raise money for the local branch of the organisation.
Miss Meecham-Flynn said plunging into the ice-cold water was the least they could do.
"We went to the hospital just after Brodie's surgery and saw how resilient he was. He's a bit of an inspiration, really.
"We deal with stress and think it's a big deal, but he'd just had his chest opened up."
Miss Meecham-Flynn admitted a lot of people laughed when they heard about the challenge.
"I don't think it has quite hit me how long it is in ice-cold water."
Mrs Cresswell said while Brodie didn't really understand the concept now, she hoped he would as he got older.
He'd be at the challenge to support them.
Mrs Cresswell said @Heart had been with them every step of the way, providing support in the hospital and in the years since the surgery, including organising special events for kids like Brodie.
During open heart surgery a child's chest cavity can be filled with an icy slush to slow the heart's oxygen requirement and give the surgeon a longer window of operating time - thus the cold water for the fundraiser.
To donate go here