The most economical trip from the Hong Kong Airport to Kowloon Area

Hi there, there are 4 of us travelling with 1 kid and a baby and we are to arrive Hong Kong at 12:50AM. Are there transportation systems available at that time to Kowloon? How much is the ticket? Also, we plan to go to Disneyland the next day. What is the most economical transportation system we can use, and how much? THanks a lot for your help! :)