We have a fairly complicated floor plan.  (at least upstairs).  We split it into a hall, three bedrooms and a 1/2 bath.  That's quite a bit for 640sq. ft.  Since I have three boys, we elected to build the walls quite strong, as if they would be load bearing (which they aren't).  This way, as the children age, the whole house should be able to withstand slamming doors, and punched walls.  Yes, these are things I would like to avoid, but if they happen, it is good to know the house will survive.