Baking soda is the most effective and safe solution to this problem. Just prepare a thick paste by mixing a few drops of water with 1 teaspoon of baking soda. Now apply it to the stained place and rub it lightly with a soft cloth or sponge. Pen-pencil marks will disappear in a few seconds. Then clean it with a wet cloth.
The special thing is that this method is not only safe for chemical-free and children, but also very economical. If there is no baking soda, then you can repeat the same process with white non-gel toothpaste.