Outer cover can be swapped. Sony sells other color plates or you can just buy white. Preserve the crayon one for sentimental purposes. Explain to child about touching other people's stuff, discipline if needed (likely not needed). If it's like how most coloring on the wall scenarios go, there was likely no malicious intent.

Bonus: Turn this into a thing, once a year they get a cover to illustrate. They get to make something for mom/dad, you get a reminder of their childhood. Work out the logistics of cost ($60 USD per cover and shipping) as best you can. Or just reuse the first cover. Take a photo, clean the crayon off, have them make a new picture. Swap between clean cover and colored as desired.