I really love this idea! Maybe the desk could have casters on it for moving out of the way to access storage under the bed.
Rustic DIY Bookshelf with IKEA Ekby Brackets. Learn how to find wood that actually fits the IKEA brackets! | A Shade Of Teal:
Possible option for "framing" the window in the play area with functional storage, would also add pillows to bench area since window is not as low, and add crown molding to make it appear more built-in
Ikea Expedit Custom Cushion You pick the by HearthandHomeStore, $179.00