Harman,
I really don't know what to advise,just wanted you to know I'm going through the same, my dd(5) is going through a stage of drawing on EVERYTHING, herself, her newly decorated bedroom walls, her carpet,her sister, the picnic table her slide and her seesaw.
The last straw came today when i went outside to bring the washing in and found that she had drawn on every piece of washing that she could reach.
Hence all the lovely dry washing is back the washer with some stain remover, I hope it comes out as some are dd2's pressies that she got for her birthday.
I have no idea why my eldest is like this and hope she grows out of it soon.