While writing an entertaining child story is obviously more art than science, most successful child stories pay attention to the following 8 elements.
A good child story has an underlying theme. The underlying theme of "Peter and the Wolf," for example, is "don't tell lies" or "be honest." The underlying theme of "The Sneetches" by Dr. Seuss is "don't be racist" or "all (Sneetches) are created equal." A theme can be the moral of the story, or an insight or viewpoint that the story conveys. Common themes are courage, love, perseverance, friendship, etc.
As an underlying theme, the theme usually emerges subtly as the story unfolds. A direct statement of the theme usually comes across as preachy and uninteresting. Remember what your high school English teacher used to say: "show, don't tell!"
Also, keep your theme positive and constructive. Your story may be sad, but make sure it's not negative, cynical or depressing!
#2: Plot and Pace
Plot is what happens in a story. Pace is the speed at which the story develops.
Generally, a simple chronological unfolding of events works best for storybooks (no flashbacks or complicated jumping around in time).
The plot usually revolves around a dominant problem or conflict which the main character must resolve. The problem or conflict may be with another character, with circumstances or even internal to the main character (e.g. overcoming their own fears).
The plot usually proceeds through phases: beginning of the conflict, initial success or difficulties, further difficulties or reversals, final resolution or victory, and outcome. As the story progresses through these phases the conflict becomes more intense and increases the dramatic tension, until it the story climaxes and the conflict is resolved.
For the most part, the main character succeeds or fails through his or her own efforts. In fact, it is through this process that the character learns or grows, and this lesson or growth typically conveys the theme.
TIP: Create a thumbnail layout/mockup of your text. This way you'll be able to better judge how your story unfolds and its optimal pacing. For more information please visit: http://www.child-stories-bank.com/writing-child-book.html.
Proper pacing of your story is essential. Too slow and the reader/listener will lose interest, too fast and they won't have time to get excited or they will miss important details. The pace in storybooks should be fairly brisk without "rushing." Avoid lengthy introductions or descriptions of the setting. Start the action immediately from the beginning and bring the story promptly to a close at the end.
Even more so than adults, children appreciate action. So, keep the pace of your story fairly quick by using action and unfolding events. Don't get bogged down in lengthy descriptions or reflections. Again, "show, don't tell!"
#3: Narrative Voice and Point of View
Narrative voice is the viewpoint from which the story is told. Most stories are told either in the "first person" (from the perspective of "I", "I did this") or "third person" (from the perspective of "They", "They did that"). If you choose to write from the first person perspective, you'll need to decide which character is the narrator. Whichever point of view you choose, make sure that you stick with it. Jumping from one point of view to another can be very confusing.
TIP: Study storybooks similar to yours or the books of a publisher you're targeting to see what narrative voice they use.
Creating interesting characters is as important as developing a solid plot. The more readers can relate to your characters, the more they will like your storybook.
So, how do you make your characters come alive? As with real people, characters come alive when they have real characteristics: personality traits, quirks, physical traits, mannerisms, a certain way of talking, fears, joys, motivations, etc. If you had to describe yourself or your best friend in a few words, what would the salient characteristics be? Try to identify one major character trait and a few minor ones for each character. Write out brief profiles if that helps.
TIP: Apart from describing them, you can also reveal the characteristics of your characters by showing how they respond to situations, or through the way they talk.
An important point is to be consistent. A character should be "true to character" in order to be believable and gain the reader's acceptance.
Set your story in a place and time that will be interesting and/or familiar.
#6: Style and Tone
Remember your primary audience: children. Write accordingly, using (mostly) short words, short sentences and short paragraphs. Write simply and directly so you don't lose your reader (also, don't forget that a lot of children will only listen to your story as it's read to them; it needs to be easy to listen to and understand like stories have been throughout time).
TIP: Before the written word, stories were transmitted orally. A good story still should "sound" good. So, try reading your story aloud. Does it flow naturally and capture the listener's attention? Are there opportunities to use your voice (tone, loudness, etc.) to make the listener feel like they're there with the characters?
Use direct quotes (e.g. "'Jump!' she said.") instead of indirect quotes (e.g. "She told him to jump."
What is the tone of your story? Is it an epic story? A funny, wacky story? An adventure story? A scary story?
Make your writing suit the atmosphere or tone that you wish to create.
Read your dialogue out loud to make sure that it doesn't sound stilted or unnatural. Does your character talk the way people would expect him to?
#8: Openings and Closings
You need to hook your reader from the beginning, so start your story with a "bang!". You want people to finish reading your child story feeling satisfied so make sure that the main conflict or problem is resolved, even if the story doesn't have a "happy" ending.