The Glasgow-based singers met young people with the disease during a visit to the Teenager Cancer Trust's facility in Yorkhill.
The children's hospital will move to the new Southern General Hospital Campus and the charity is fundraising to build a new unit for teenagers in the west of Scotland.
Lewis Andrew from the band said Kassidy decided to support the Teenage Cancer Trust based on personal experience with the illness.
He said: "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."
The Trust's unit in Yorkhill Hospital has beds for six 13 to 16-year-olds.
Once cash is raised the new state-of-the-art unit will provide six inpatient bedrooms with en suite bathrooms and have a social area with TV and computer games.
There will also be computers, wi-fi and a kitchen where young people and their families can prepare meals.
To donate see www.teenagecancertrust.org or text GLA15 to 70500 to donate £5.