Hi There,
The type of pen is important.
If it's a ball point pen then the answer is different to if it was a Texta or Permanent marker which would be different to a white-board marker whi ... I could go on, but you get the idea.
To cover the most common ones:
On the Cookbook:
White-board marker: Hi-Gloss pages might allow you to wipe it off if you're quick, otherwise an artistic approach with liquid paper might help?
Ball Point Pen: An artistic approach with liquid paper might help?
Permanent marker: An artistic approach with liquid paper might help?

On the doll:
White-board marker: Cloth with a dash of metho, allow doll to dry properly before littleys can lick it.
Ball Point Pen: 120 Grit abrasive paper
Permanent marker: Angle Grinder
On the Child:
White-board marker: Time ... Wash well with mild soap each evening & eventually it's all gone
Ball Point Pen: Time ... Wash well with mild soap each evening & eventually it's all gone
Permanent marker: Time ... Wash well with mild soap each evening & eventually it's all gone