I am trying very hard to keep the walls at home, as clean as possible by allocating the wall in my kids room, to be a grafitti wall. They have all the freedom to do what ever they want to the wall.... this is a very BIG mess.
You know, too much of restrictions is not good and the kids need to explore and build some self esteem.
As you might propably guessed, I am at the losing end... but still, when they grown up... I will have the momories of these! Of course, during some art attack session, any wall comes handy!
Here is the drawing of a fire engine...

.... in my room!
An abstract picture.... what do you see?

When they grown up, I will make them paint the wall...! Daddy will provide the paint!
..