Looks really good.
Just one thing you should consider is Piaget's 5th equation of cognitive development;
Child + hidden compartment = Child in hidden compartment*
Do assess the ability for your grandson, at various ages, to get into the storage box, and then get out again, or at least breath and raise the alarm. I don't mean to be a downer here, and you may have already considered this, but we all need to look at our projects from a safety point of view.
Jeremy
*ok, I just made that up, but you get the point.