Adjoining rooms are just rooms that are next to each other. Sometimes, they may not even be right next to each other. They may or may not have a door inbetween them that connects them together.
So, what you want to ask for is Connecting rooms. And, they should guarantee them if there are 2 adults and 3 or more children. If you have 3 adults and 3 or more children, they won't be guaranteed, because you could have mom and dad in one room with 2 kids and grandma (or whoever the other adult is) in the other room with the rest of the children.