William Cunningham and Jacob Sharp starred for Havant among top-class competitors in Crawley.
Andy Vincent and Alfie Cunningham also performed well as the club secured a hat-trick of gold medals.
It was William, seven, who led the way as he picked up gold medals in both kata and kumite in the purple belt section to be crowned grand champion.
Sharp, 10, secured a gold medal in kumite and a bronze in kata in the orange belt category.
William’s older brother, Alfie, 14, got to the final of the black belt kata to finish in fourth place.
Meanwhile, Vincent, 47, fought well but lost to an England squad member.
Havant karate coach Steve Fife said: ‘I am very pleased with Jacob’s performance in his first competition.
‘He has only graded once – to get bronze and gold is a great achievement.
‘William was outstanding, he even outshone his older brother, Alfie, with two golds in both sections.
‘To be a grand champion at the age of seven is superb.
‘He’s an England squad member in the making.
‘Alfie did well to get to the final but it is very hard to maintain his top level karate.
‘Andy fought well throughout the day.
‘Overall I was very pleased with the performances and a hat-trick of gold medals.’
Alfie Cunningham will be representing England in Norway later this month.