I suggest you paint the walls, including the underside of the staircase, in one of the whites shown in the clouds on the ceiling graphic . You don't want busy colour on the walls in a tiny space. The fact that the art you propose to hang is in white frames, is great. The children's toys will add all the splash of colour you need. I like the green matting/carpet, because it reminds me of lawn. Can it be stretched, and hammered into place so it is neat and tidy, like freshly cut grass?

Buy grassy green baskets/containers to throw all the toys into after play time. Alternatively, buy white. The key is to keep to a limited colour palette on the walls and floor, because the toys will add the colour punch.

Donna White Interior Design - Classic Design with a Twist

Great thanks! love the ideas