If a kid’s room is located in a small area that has only a bed and a desk and you have an empty area under the stairs, why not turn it into a playing area for kids? Kids like when they have a private space like a house with a separate entrance. Also you can use this design idea if you have children of different ages, because for younger children it is more comfortable to play in other room without hindering older children. You can design the area under the stairs as a reading nook or a whole playhouse. It is up to your inspiration and desire.