Year 6 student Sienna Dy-Mortimer has been selected to have her rainbow design printed on Crazy Arms, the sun-protection sleeves that are sold online around Australia and the world.

Crazy Arms were created by three Sydney dads who experienced the daily battle of getting their kids to wear sunscreen.

The slip-on arms are worn under T-shirts or school and sports uniforms to help protect kids against UV rays that can lead to skin cancer.

The arms' UPF 50+ lightweight material is made from old fishing nets recovered from the ocean, and other nylon waste.

The Crazy Arms are manufactured in Australia and their designs include skulls, mermaids and unicorns.

Forrest Primary mum Cath White, whose husband Ian sadly passed away in 2016 from a brain tumour after a battle with melanoma, bought some Crazy Arms for their daughter Alexandra, now 7.

She then asked Crazy Arms to sponsor a design competition to support Forrest Primary's sun-smart policy.

Principal Chris Pilgrim said it took "a long time and many cups of tea" to pick the winner from all the fantastic entries.

Sienna won with her rainbow-inspired design. "I really liked rainbows and I just added arms," she said.

The three dads - Martin Rippon, Stephen Busuttil and Jason Rance - were originally motivated when Martin got a skin cancer on his nose.

They only launched last year and have already had orders from around Australia, as well as Germany, Hong Kong and New Zealand.

Martin said Sienna's design was "absolutely striking" and would go on sale in a limited-edition run this summer.

Crazy Arms would also be using more kid-created designs, he said.

"We were so impressed by the designs that the primary school kids came up with," Stephen said.

"While skin protection is a serious business, we want Crazy Arms to be a fun product to wear and what better way than asking kids to make the designs themselves."

The kids were also entertained at the awards assembly by Sid the Seagull from Cancer Council ACT and his friend Belinda Barnier, executive manager of community support and engagement, who said "clothing was king" in sun protection.

Cath White was thrilled with the result.

"Forrest Primary is an [International Baccalaureate] school and it's about action. I'm just so glad the kids got to see first-hand what a couple of dads could do to make a difference," she said.

Crazy Arms also donates $2.50 for every dolphin design it sells to healthyseas.org "to help keep up their good work clearing the ocean".

This year, Crazy Arms is launching a sports club/affiliate marketing program to help protect kids on the sports ovals. By signing up, the clubs can also use this to raise funds, with $4 from each sale going back into their funds.

"It's all about being part of the community, and having fun," Martin said.