A theme is an insight or viewpoint or concept that a story conveys. If an editor says your story is "slight," this may mean you have no significant theme.
Don't blurt out your theme. Let it emerge from the story. If you must come out and say it, do it in dialog, not narration.
Avoid preaching. Children's stories should be explorations of life, not Sunday school lessons.
Keep your theme positive. If writing about a social problem, offer constructive ways for your readers to deal with it.
Plot is normally built around a conflict involving the main character, for instance, with another character, or with circumstances, or within him or herself. A story may succeed without conflict, especially if for preschoolers, but not without another device to hold attention.
Conflict often takes the form of a problem the main character must resolve. The character should succeed or fail at least in part through his or her own efforts. Most often, especially in realistic fiction, the character learns or grows in the process. The lesson or growth conveys the theme.
The conflict should result in increasing dramatic tension, which peaks or "climaxes" towards the end, then resolves.
The basic sequence of plot stages is: arrival of conflict, initial success of the main character, reversals, final victory, and outcome. The success-reversal sequence may repeat.
A novel may have several conflicts, but a short story or picture book should have only one.
Move the plot forward with events and action, rather than with internal musings. Show, don't tell.
At the beginning, jump right into the action. At the end, bring the story to a prompt close.
Keep the structure as simple as possible. In a picture book, keep the action in chronological order without "flashbacks" (insertions of earlier scenes).
For a picture book story, make sure you have enough "scenes" (locations) to provide variety in the illustrations. For a magazine story, on the other hand, don't have too many, as space limits the number that can be illustrated. The number of scenes determines whether a story is best suited to a picture book or a magazine.
For your narration, make the best choice for your story between "first person" and "third person." In first person, the story is told by one of its characters, "I did this." In third person, it's told as if by an outside observer, "They did that." First person is popular with middle-grade and young-adult readers as it creates instant intimacy and can convey lively wit and emotion. But it can confuse younger listeners, so it should seldom be used in early picture books. Third person is fine for any age, and permits the writer more sophisticated language and observations.
Whether in first or third person, the story should generally be told through the eyes of a single character -- usually the main character. This is called "point of view." Sudden shifts in the story's point of view can jolt and disorient the reader. To keep it consistent, narrate only what your chosen character would know and nothing he or she wouldn't -- for example, other people's thoughts, or something out of sight. If you do need to switch to a different point of view, set up a separate section or chapter for it.
Before you start writing, know your characters thoroughly.
Your main character should be someone the reader can identify and/or sympathize with. He or she should be near the top age of your intended readers. (One exception is in folktales.)
Identify your characters with one or more telling details, a physical trait, a mannerism, a favorite phrase. A complete description is not needed.
Set your story in a place and time that will be interesting or familiar.
Style and Tone
Write simply and directly, in short words, short sentences, short paragraphs.
Use dialog wherever possible. Use direct quotes instead of indirect. (Example: "Go away!" instead of "He told her to go away.") Aim to make dialog at least one-third of your story.
Avoid big chunks of narration, especially description. Often you can split it into smaller pieces, or convey information in dialog. (Example: "I like your purple hair.")
Use language that creates an atmosphere or "tone" suited to your story.
For younger children, use poetic devices like rhythm, repetition, alliteration ("Peter Piper picked a peck"), and rhyme though generally not in verse.
Avoid being cutesy or sweet or sentimental or condescending.