Evaluate the target age groups for different childrens' books, then decide which age group you want to develop the plot for. As bestselling children's author Robyn Opie suggests (see Reference 1), you should read as many books as possible that are proven favorites for your target age group, because the similarities between them will also showcase the reasons why they've been successful. Make detailed notes in regard to these similarities, as well as concepts and ideas that are of interest as a starting point for further original plot lines. Detail all notes including character development, plot, and even imagery.
Develop a main character. Children often respond to characters in whom they can see themselves, especially if the target group is four years old and up. Little girls or boys who hold similar characteristics to the young readers or to someone they know, will be more attractive to these readers in a story than those who are so outlandish, that they're hard to relate to. Build on the ideas behind the main character including their appearance, traits, where they come from and their family life.
Introduce secondary characters such a family members, friends or pets. Refrain from creating too many characters for a young child's story as that will make it overwhelming for them to follow. Focus on only a few in addition to the protagonist. Establish the relationship between these characters and the main one and make it clear how they're connected, so the reader does not get confused.
Make notes of various problems that could arise for the protagonist that can be solved within the confines of their abilities. For instance, if it is a small boy and his dog goes and digs up the garden, the boy must then find a way to clean up and wash the dog. This would be a big job for a young child, but it is something they will recognize as a problem. Jot down as many problems as possible and then weed out the unsolvable ones until a realistic one stands out.
Integrate the main character with the problem, then allow the secondary characters to come and help solve it. Resolve the problem in a fun and interesting manner where teamwork is key, as children are taught from a young age to work together and share. Plot a timeline for the events and add in any anecdotes that may fit with the overall storyline. Ensure the length is appropriate for the target audience and then focus on imagery to conduct the concepts behind the story.