"Adonai and Savanah are cousins, but they are more like siblings and do everything together.
"Amelia didn't want to do it at first, which is why she ended up being a separate registration - but after she heard her cousins were doing it too she didn't want to be left out."
She said the children were taking part because it was fun, but for her there were more benefits to the triathlon than just enjoying a day out.
"It's about keeping the kids active and off their devices.
"The kids really love the water, TJ especially is a real water baby, and has just started waka ama, so I think they'll get a lot out of the triathlon on Sunday.
"With summer coming up I want them to enjoy spending time outdoors and getting into sports."
Sanitarium's national marketing manager Jessica Manihera said it was great TJ was inspiring others, including his own family members, to get involved in the event.
"It's super to hear TJ enjoyed the event so much last year he came back again, bringing others with him - that's exactly what we're trying to achieve with the Weet-Bix Tryathlon.
"Just getting involved and participating, giving it your best and receiving a medal for your efforts - that's what it's all about."
The children will be cheered on by family and friends, as well as locals who want to encourage children to get out and active.
Cowley-Ross said she loved sport and the feeling of challenging yourself.
Jones said one the best parts of being involved in events like the triathlon was the people you met along the way.
"I couldn't have done what I've done without the support of family, friends and those around me, and at the triathlons you're surrounded by people who just want to see you try your best."
This year's series marks the 25th anniversary of the Sanitarium Weet-Bix Kids TRYathlon and looks set to be the most successful ever, with entries across New Zealand up 42 per cent on last year.
Visit try.weetbix.co.nz for more information.
What: Weet-Bix Kids TRYathlon
When: Sunday 9am
Where: Kuirau Park
To enter: Online registrations are now closed. Register on the day from 7am to 8.15am for $56.