Me and my daughter wanted to go to gulangyu island during our 12 hrs lay over in xiamen airport.

You can take bus no.27 at Xiangyun 1st Road Terminal 3 Bus Stop to the last stop, No.1 Port at first. The ticket fare is CNY1/person. Upon arrival, you can take a ferry to the island directly. As I know, you can follow this route to visit the island.
Sunlight Rock – Shuzhuang Garden – Haoyue Park – XM Underwater World – Organ Museum – Trinity Church
All of these attractions are on the island, which you can walk from one to another.
If time permits, you can also take a taxi to the South Putuo Temple and Xiamen University.