I see it's an X series, but does it have an HPL top or is the top solid wood?
If it's HPL, you should be able to clean it with something like Fantastik like someone said before. If it's a wooden top, the art gum type eraser, (something very gentle), would be my first try.
But I have to admit, while I wouldn't want my grandchildren to write or draw on my guitars, the idea of leaving it as a memento is a pretty neat idea. I also love the idea of handing down the guitar to him someday.
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