"We have done 4- and 5-year-olds before, very little kids. But having instructors available to take the classes during the day was difficult," Mr Davies said.
The after-school classes involve correct movement exercises using athletics and gymnastics skills through activities and games. These include squatting, throwing, lifting, pushing, pulling and jumping.
"We focus on how to move. We teach them how to do a squat, or how a throw is done."
Mr Davies, a former personal trainer and certified crossfit coach, said the 40-minute classes were proving beneficial particularly for children who previously had low confidence when it came to sport.
"The parents are amazed at the high number of kids who have no interest in sport but they've grasped crossfit and then ... start playing sport or run in the cross-country when they've often avoided it."
He acknowledged there was risk of injury but said that was no different to any other sport.
"Any fitness programme can be dangerous in the wrong hands but we don't have a focus on loading heavy weights. The kids don't do the same crossfit as adults do."
Mr Davies said while the children did sometimes lift kid-sized (4kg) weights and backpacks, the focus remained on how to do the exercises correctly. "To this date we've not had any kind of injury."
Children are taught the "angry gorilla stance", standing and squatting like gorillas to stay safe when lifting something off the ground.
The manager of Project Energize, a healthy eating and activity programme for Waikato school children, said she would be horrified if the classes were too similar to the adult version.
Sport Waikato active communities manager Stephanie McLennan said CrossFit Kids at Birkenhead sounded appropriate but she believed very young children should stick to running around outside.
"For 3- to 5-year-olds it's more about spontaneous play rather than directed activity. Crossfit requires a high level of aerobic capacity and children can't work anaerobically."
Mrs McLennan said young children physiologically did not have enough breath to exercise at an adult level and they should not be exposed to highly competitive sports.
She called gyms an "unnatural setting" for playing games and most Kiwis could not afford to pay for play.
Fitness and fun go together for Piggin family
Self-confessed "crossfit junkie" Matt Piggin says his love of the exercise regime has rubbed off on his two young daughters, who now attend crossfit Kids classes.
Riley, 6, and Piper, 3, joined the class four months ago after learning some of the training techniques at home with Dad.
"They do enjoy going and they look forward to it.
"They run around playing silly games and having a whole lot of fun."
Mr Piggin said that at the girls' young age the weekly classes were no different from doing other sports activities including swimming, soccer and gymnastics.
The 40-year-old Birkenhead company director said Riley and Piper were naturally athletic but going to crossfit had encouraged their climbing and gymnastic abilities.
"They're legs ahead of other kids in terms of monkey bar climbing and hanging. The confidence they have is quite huge ... especially the younger one. She can do backflips on the rings."
Piper, who will be 4 next month, is also involved in a ball skills class at kindergarten and Riley will begin netball next term.
Mr Piggin said he and his wife, Cass, wanted the girls to be physically active.
"I grew up playing outside, not sitting around watching TV or playing computer games, so we're trying to keep the kids active now and get them into sports and things that hopefully sets them on the right track for the rest of their life."
His advice to other parents considering crossfit for their children is to make sure they have a knowledgeable instructor.
