My 17 month old decided to use our wall as her art board with non-washable crayons. It is painted with a matt finish so I thought it would never come out. Well I tried everything I could think of. Then I combined two tips that I saw into one.
I used baby wipes with baking soda. I couldn't believe it, but it worked. I had to use some elbow grease, but after awhile, it was all gone. I have now switched to all washable crayons for both our girls.