Bus to take a family of 4-2 kids one is 4 and other is 1 from HKIA to royal garden hotel?

Bus is the cheapest way. You can take no. A11 at HKIA and get off at Mody Road, Chatham Road South. Your hotel will be in the east. In this way, each adult needs to pay 33HKD and the kid of 4y/o enjoy 50% off. The 1 y/o kid can take for free. It may take around 70min.

Another option is that you take Airport Express Train to Kowloon Station with the group ticket for 3, which costs 210HKD. Then transfer to free shuttle bus line K3 and get off at Regal Kowloon Hotel. Then walk to your hotel in 3-5min. The duration can be 40-45min.