Leyette Callister, principal of Howick Primary School and president of the association, said swapping players between teams was one way to ensure all children enjoyed their earliest sporting endeavours.
The charter was being introduced because children could be devastated if losing margins were too huge.
"We want winners to win, but we also don't want [children] to be absolutely annihilated so that they give up. We want them to keep doing it."
One issue was some schools had far larger rolls than others, giving them an unfair advantage.
"There are a lot of smaller schools that possibly wouldn't enter [competitions] at all if they knew they were just going to get smashed," Callister said.
"We're careful with our draws but this is about ensuring games are as fair as possible. It's about the kids going out and having a game, not about winning."
The charter was not mandatory and would be at the discretion of coaches, referees and other officials.
The charter also provides parents and spectators an avenue to complain about the refs.
"Sometimes people get a little bit excited and have their own opinions and they're absolutely entitled to them," Callister said.
"But I have heard of parents who have rushed up to [the referee] at the athletics day with their camera saying, 'I took a photo of my child and it shows they actually came in third not fourth and therefore you've made a mistake'.
"So this gives parents who dispute the call a pathway to go through."
The charter states that sport in schools is there to encourage students to be active, with clubs providing a higher level of competition.
It was important that players, coaches, officials and parents all bought into the spirit of fair play. Those who failed to could "at the discretion of the organiser" face "appropriate action".
Going easy on the underdogs
• The so-called "mercy rule" is most common in American sports such as baseball and softball where there is no game clock and play could theoretically continue forever.
• In 2012, Auckland soccer organisers were accused of "living in cloud cuckoo
land" after allowing children's teams to share championship titles.
• In 2011 the Rugby Union introduced rules to prevent children's teams winning by more than 35 points.