Welcome SB2.
I guess if it is a family of 3 (including a toddler) they would offer you one room only. Your son is not charged if he is under 2-year-old. Well, I m not sure if that kids club does not allow for any kid below 2 to join strictly since their activities are some story telling and finger print. (but it is charged at around HKD300 per night as I rmb.)