It is an appealing question. But, do you really have to put something on a wall? If you see at the historical publications from the history, you will notice that they have continually put something on their wall space, even when people lived in a cave, they painted their wall surfaces and put many simple decorations including plants.
Children often have their own universe – the land of dreams and imagination. Although it could be not easy to enter this zone, the adults can help their kids to realise their fantasies, at least in some part.