Did you try an eraser?
I would try a cleaner like Fantastik on a cloth (do not spray directly on the guitar) to remove the graphite. Just dampen the cloth with the cleaner and rub the marks gently. A polishing compound will remove the marks but will leave a shiny spot on the satin finish.
If your grandson had a heavy hand, the pencil will leave indentations that might lessen with judicial application of steam (causes the wood to swell and push out). If that doesn't work and you want to restore it to "new" condition, bring it to a luthier who can sand down the area and blend in a touch-up finish.
All of my guitars eventually end up with a few bumps. nicks and scratches, so if it isn't too big of an area, I would let it be after removing the pencil graphite.