A very adequate bedroom can be made in a seven foot square area. Many options are available to provide storage and floor space by using bunk beds and the area under beds. Very often we build houses with huge bedrooms. It is very difficult to justify a larger bedroom unless the bedroom is also to become a game room or a play room. It should be remembered that bigger rooms lead to bigger messes.
Bedrooms can easily become too large for children to manage.
Also some parents insist children play in their rooms, but young children want to be with their parents. Many times I will have young mothers tell me they want their children to play in their room and keep their toys out from under foot. Inevitability, the child wants to be in the room with their mother. If she moves into the kitchen to work, that is where the child will go. If she moves to the laundry room, so will the child.