Create a mini apartment with beds PRIVATE areas SEPARATED each other also for the age difference between the kids, and the studying/living area is in the middle (useful for a future division). No demolition needed.
Functions
BATHROOM
I suggest to create a first washing area, to have a double use of the bath for the kids and have more privacy.
GIRL'S BEDROOM
I create a provate space for bed area, a simple private corner, with a make up/walk-in closet area. This area is also dividable with painted/wallpapered sliding doors (see the arrows).
RELAX/CONVERSATION AREA
A sofa-bed to have friends or guests in the middle; ready for the future.
KITCHENETTE/STUDY CORNER
Here again I suggest the use of sliding barriers, such as doors or glass doors...
BOY'S BEDROOM
A special nest for this kid!
WALK-IN CLOSET
Using this useless corner to create a double walk-in closet.
One for the girl measuring :
1,32 x 0,90 meters.
One for the boy :
1,00 x 0,80 meters.
Two wardrobe cabinets closable and very useful: order guaranteed.