After arrive at Hong Kong Airport, there are directions guiding you to take the Airport Express train, by which you will arrive at Kowloon train station in 20 minutes. The fare is HKD90/adult and HKD45/child(3~11 years old). Then, you could find the free shuttle bus K5 which drive you from Kowloon station to Royal Plaza Hotel directly.

Actually, since you have 4 together, I think it is also reasonable for you to take a taxi from the airport to the hotel directly. The fare should be around HKD300.

To Disneyland(https://www.travelchinaguide.com/attraction/hongkong/outlying/disneyland.htm) , you could take metro from Prince Edward station to get there directly. The fare is HKD18.5/adult and HKD9/child. The Hong Kong Metro fare is the same at weekdays and weekend.

To Ocean Park(https://www.travelchinaguide.com/attraction/hongkong/island/ocean_park.htm), you could take the metro to Admiralty station, and then at Exit C, there is the bus 629 driving you to the Ocean Park directly. The metro fare is HKD10.5/adult and HKD5/child, the bus fare is HKD10.6/adult and HKD5.3/child.