Have all of your students answer the same question, or write on the same subject. Topics such as favorite foods and places, dream vacation destinations or life ambitions make good prompts. Alternatively, write a sample fill-in-the-blank sentence and have each student fill in their individual answer to begin their section of the project (for example: "My name is ________, and if could have any superpower, it would be _____________.") Each student should write a single page; make illustrations optional so your children are not overwhelmed.
This project is ideal if you want to teach your students about writing over a longer period of time, and you should ideally start it at the beginning of a semester or school year. One day each week, have your students write a journal entry on a specific topic or the things that they have done over the past week. At the end of the semester, have each student pick their favorite entry from their journal. Compile these entries into a book, and have each student illustrate their entry.
Use the classic mad lib game to write a book with your students. This makes an entertaining activity in and of itself. Before class, write a story about the length that you wish the class book to be, being sure to leave strategically placed blanks to fill in later with verbs, nouns, and other parts of speech (this game is also a helpful way of teaching grammar to your students.) Before showing the story to your students, ask them what words they wish to fill in the blanks. Make sure that each student gets an opportunity to fill in some of the blanks. Once all the blanks have been filled in, read the story aloud to your students. You can then make the mad lib story into a book, having each student illustrate several pages.
The exquisite corpse is an old literary game that you can adapt to a classroom setting. Begin by having one of your students write two lines of a story (preferably on lined paper.) Fold over the top line so only the bottom half is visible to the next student. Pass the paper around the class, with each student adding two sentences and then folding the paper over so only the bottommost sentence of the story is visible. Once every student has contributed, unfold the paper and read the story aloud -- the results should be entertainingly absurd. If you are making the story into a book, have each student's section be a page, and have each student illustrate the section they wrote.