For a book to really succeed, it has to have universal appeal. This means readers from all walks of life need to be able to relate to your story at an emotional level. In other words, they need to care about your story. For children’s picture books, this applies to both little people (the listeners) and big people (the readers and buyers). It seems obvious, but it’s often an overlooked fact that if people don’t care about the subject matter of your story, they’re not going to want to read it.
A problem that many beginning writers have is that they’ll write stories that have significant meaning to themselves, their children, or family members, but outside of that small circle it’s just a nice story. The story has to have significant appeal to a very large circle of people that encompasses the country, and better yet the world. That doesn’t mean that it has to be a story of interest to everyone in the world, just a large cross-section or percentage.
However, a word of caution. Many of the subjects with the greatest universal appeal have been so overdone that they have become cliché. Unless you can present them in a very unique or fresh fashion, you should stay away from these subjects.
So as part your of writing efforts, you need to search for subjects that have universal appeal. For children’s books, you have to discover what kids care about.