Ruby, 11, from Campion College, will compete in the Grade 11 girls' 100 metres and 200m.
It will be her second Colgate Games.
Her elder brother Jack, 14, will compete in his third and final Colgate Games and is a strong chance in the Grade 14 boys' 200m.
He is also expected to be among the contenders in the 100m and 400m.
Jack said he made friends at previous meets and it was “cool to meet people”.
He was a student at Campion but got an athletics scholarship and will attend King's College in Auckland this year.
It will be the third Colgate Games for Evelyn, too, and the Gisborne Girls' High School student has entered the Grade 13 girls' 100m, 200m, 400m and 800m.
She had a podium finish in the 800m last year.
Evelyn, 13, said she enjoyed having the chance to compete against more athletes than usual in her age group.
Kaedyn, 10, from Mangapapa Primary School, is new to athletics and showed potential at last year's Gisborne inter-school championships. He won four titles in his division.
In Taranaki, he will compete in the Grade 10 boys' 100m, 200m, long jump and high jump.
Organising committee chairman for the North Island event Simon Elliott said it would be an exciting three days of competition in a big year for the sport.
“With the Tokyo Olympic and Paralympic Games just a few months away, it's great to see the positive enthusiasm and energy from young athletes, coaches and parents as we head into such an exciting time for the athletics community.”
Athletics New Zealand chief executive Peter Pfitzinger said participation in the North and South Island Colgate Games had remained strong over their 42-year history.
“Athletics is a foundation sport that teaches movement skills that are transferable and, with such a range of events to choose from, there really is something for everyone.”
Athletes at past Games have included Tom Walsh, Nick Willis, Jeff Wilson and Bernice Mene, who all went on to big things in athletics or other sports.
The youngsters at the North Island event this year will represent 87 athletics clubs.
Jack and Ruby's father, Paul Sadler, said the Colgate Games had retained a theme of being both competitive and fun.
“It exposes athletes from different regions to more competition. And they make new friends.”