The 10-year-olds from North Parramatta like most things outdoor and sporty.

Hamish rushed back from a soccer camp for a run with his sister on Friday.

Now they'll be swimming, running and cycling as part of an attempt to set a world record for the world's biggest kids' triathlon, the annual Sanitarium Weet-Bix Kids 16s series, which starts at Sydney Olympic Park on January 31.

The twins aren't new to the caper: this will be their fourth go but the triathlon is different for another reason.

No times are taken, no finishes recorded, everyone gets a medal and the distances vary for the seven- to 15-year-olds eligible. The aim is to get kids off the couch and exercising, setting habits for a healthy life.

The series runs until April and is expected to attract 14,500 Australians.

Registrations close this Sunday at weetbix.com.au.