I would do the boxes next to each other (or shelves for art supplies & books), add a cushion on the boxed (shelf area) with textured pillows. Then add a leg to support the desk on the end with two chairs.
DIY Washi Tape House from Poppinette. This is such an easy and cute DIY for a kid’s room. A Personal Experience Tip: Try a small piece of tape in an inconspiculous place to make sure it doesn’t rip off your paint.