Similar to hotel room, standard cabin of a HaLong Bay cruise is double or twin room. In principle, such a room allows a maximum of 2 adults and 1 child. So when your team is bigger, cruises hardly accept to put you all in one cabin, even if it means 2 parents and 2 children.
Many cruises companies offer triple/connecting cabins but not all the boats in the fleet have this type of cabin. Also there are only 1 or 2 cabins like that per boat, so somehow you can say that the choices are still limited. Good news to know is that the cabin will be relatively spacious and comfortable and the rate per person is similar to standard cabin.
