Sanding is right--I would sand even if it's not gloss already, then prime to keep the paint from chipping.
Before you paint, though, have you tried the Magic Eraser? My son wrote on everything--walls, banisters, furniture--with everything from markers to pencils to crayons, and that tidy little eraser took every bit of it off.
If you've tried that already, though, and it didn't work, do try to match the paint carefully. All whites are not the same, and paints range from flat to gloss. The closer you get it, the less work you'll have to do.