Hong Kong Octopus Card , ferry to Macau etc.

If we buy the card from airport , can we use it start from airport by Airport Express Train ? How much is Octopus Card for child 4 years old ?
Can we also use it to Disneyland ?
By the way , if we want to go to Macau , understand we do not need visa ( for visitor from Thailand ) . How much is ferry to Macau and price for child 4 years old too? Is there any pier to Macau in Tsum Sha Tsui area as we will stay somewhere at Nathan Rd.
Also , what is the weather like during Dec.15-17.
Thanks and Regards,
Palinee