I found a few ideas for their mark. Based on their hobbies :) 
[img]https://kickassthings.com/wp-content/uploads/2017/12/illustrative-book-tattoo-2.jpg[/img]
[img]https://i.pinimg.com/originals/50/63/d8/5063d8a7ff794ba7f5b7fef41482b9c5.jpg[/img]

Or just symbolic/cute design I like
[img]https://www.entertales.com/wp-content/uploads/Untitled-design-1-23-1280x720.jpg[/img]