Moortwon Primary pupils Zack Angadi, 11, and Lennox Stevenson, 10, will complete a half marathon in 5k relay stints around the lake at Roundhay Park in Leeds from 12pm on Saturday.
The boys came up with the fundraising idea after Lennox’s grandfather David Garnett died in March after suffering from cancer of the oesophagus.
Lennox’s mother Paula Stevenson said: “We are really proud that they have come up with the idea themselves.”
Zack’s mother Jo Angadi, said: “Zack does the 5k park run at Pontefract Park every Saturday. They are both really competitive and are really looking forward to the run.
“It’s a lovely thing at their age to be thinking of others.”