Hi! I am planning to have a stop over in HK for about 12 hours only.

Yes, 12 hours are enough for you to visit Hong Kong Disneyland.
You can take airport express to Tsing Yi Station first. Then take Tung Chung Line to Sunny Bay Station and transfer to Disneyland Resort Line to Disneyland Resort Station. After arriving, walk to the park directly. It will take around 45 mins on the way.

To get back to the airport, you are suggested to take a taxi and the fare is around 110HKD.

Thanks!