I don't have kids, but it's my understanding that the smaller ones cannot be trusted with pens or markers. (Why, after all, were washable markers invented?) Turn your back on a pen-wielding toddler for a few minutes, and marvel at all the new places that have been marked up. The tattoo jobs inflicted on unsuspecting dolls can be particularly alarming in appearance... especially if the doll doesn't actually belong to the kid in question.
Baby Toolkit has an interesting tip that will help you get rid of pen marks on dolls without damaging any of the marked surfaces (the way some solvents, or even excessive scrubbing, might). After trying many, many stain-removal methods, Adrienne finally had success with 10% benzoyl peroxide, the active ingredient in a lot of acne medications. When left in direct sunlight, it broke down the dye in the ink within a few hours. The doll itself was not bleached.
Granted, sunlight is in short supply in some parts of the US at this time of year. It's still worth a try: UV light is UV light, whether or not the sunlight is strong and warm. This is suggested as a good idea for cleaning up collectible thrift-store purchases, but I think it'll probably help with post-Christmas sibling fights, too.
[via BoingBoing, where the comments addressing why this works may be illuminating.]