Identify a subject that lends itself to pictorial representation. Storybooks are light on words and heavy on pictures. Even complex topics can be presented if they can be adequately illustrated. For example, even DNA replication and transcription can be illustrated in a way that makes sense of putting building blocks together.
Immerse yourself in the subject to reach a level of competent understanding. Only a small percentage of your knowledge will make it into the story, but it needs to be the right percent. The reader is counting on you to put in the most important facts and leave out those that are less central. For example, the fact that rare earth elements are created as electrons and added to inner shells is not nearly as important as the fact that electrons are in a loosely defined cloud surrounding a nucleus of heavier particles.
Find the goal and the barriers to reaching the goal. There is no story unless a character wants something and there is something that keeps the character from reaching that goal. Luckily, the goal of understanding the way the world works is a constant in science, and there is no shortage of obstacles to scientific discovery.
Connect the search for the goal to personal characteristics and emotions of the scientist. Interest in a story is heightened when readers can empathize with the main character. For example, William Perkin invented the first synthetic dye and started a whole chemical industry, but he was trying to impress his professor at college and also convince his father that a career in chemistry was not a waste of his potential. Bringing in the pressures he was under adds a new level to the reader's involvement.
End the story with success. Everyone --- particularly children --- likes a happy ending. The search, the challenge, the discovery, along with motivations and emotions, create a compelling story, but only when the challenge is met. End with the triumph of discovery and you'll have readers coming back for more.