Three members of the Glasgow-based group met young sufferers during a visit to the Teenager Cancer Trust's facility in Yorkhill.
With the sick kids' hospital moving to the new Southern General Hospital Campus, the charity must raise cash to build a new unit.
Lewis Andrew from the band said Kassidy decided to support the Teenage Cancer Trust based on personal experience with the illness.
See our exclusive video of Kassidy performing at Yorkhill below.
He said: "Being local we're really proud to help Teenage Cancer Trust launch this appeal. When my cousin was diagnosed with cancer at 19 we were all in shock. No-one expects a teenager to have cancer.
"I know how incredibly important it is that young people have these specialist services to help them through the toughest times in their lives.
"We want everyone to help get this new unit built for young people with cancer in the West of Scotland."
Teenage Cancer Trust (TCT) is the only UK charity dedicated to boosting the chances of survival for young people with cancer.
Currently, the Trust's unit in Yorkhill Hospital has six beds for 13 to 16-year-olds.
The facility gives patients the chance to mix with other people their own age and stops them becoming isolated on wards with much younger or much older patients.
Andi McNicols, from Pennilee, was diagnosed with acute lymphoblastic leukaemia in January last year.
The 16-year-old had a gruelling nine months of chemotherapy, involving more than 400 treatments, and now visits the hospital once a week for top-up treatments.
Andi, a Rosshall Academy pupil, said: "I was in isolation for five weeks and I wasn't able to see anyone or talk to anyone about what was happening to me. When I came up here I made some really good friends and we were able to talk about everything together - I got a lot of support.
"I have been on children's wards and there's nothing for people my age. Here we've got Sky TV and wi-fi. It makes you feel a bit more normal."
Andi's mum Sharon added: "Andi's brilliant and we take our lead from her.
"But when she was first diagnosed she wasn't really facing up to her cancer.
"Being in a specialist unit with other people her age and with specialist staff helped her cope and come to terms with what she was going through."
Once cash is raised the new state-of-the-art unit will provide six in-patient bedrooms with en-suite bathrooms and have a social area with television and computer games.
There will also be computers, wi-fi and a kitchen where young people and their families can prepare meals.
To donate to the campaign see www.teenagecancer trust.org or text GLA15 to 70500 to donate £5.