I found the perfect book for teaching assertiveness! I was in our school library and stumbled across the book There Is a Bird On Your Head! The book is hilarious! I use Second Step in grades K-2 and one of the skills for learning is Be Assertive so I used this book with my first graders and they loved it. I also created a worksheet to go along with the book and had each child share their completed worksheet with the class. If you don't teach Second Step or teach in the classrooms this book is still great for teaching assertiveness in individual or small group sessions. So many of my referrals list "doesn't ask for help when needed" or "pouts and sulks when work is difficult" as a concern so I do a lot of work on assertiveness. I would check your school library for this book as a lot of schools seem to have the Elephant & Piggie Books. If not you can purchase it on Amazon for $6.00.
I have students draw a picture of themselves with a bird on their head and then write on the lines below: