Children ask enormous questions. Why is the sky blue. Why do some frogs freeze. Why does music give you chills. These make wonderful picture books and terrible ones, and the difference is almost entirely structural.
The terrible version is a list of facts with pictures attached. It is accurate, it is well-meaning, and no child asks for it twice. The good version has a shape — and the shape it needs is not the shape of an explanation. It is the shape of a story.
Start with the surprise, not the topic
Do not open on the question. Open on the thing that makes the question worth asking. Not "have you ever wondered why some frogs freeze?" but a frog, in ice, apparently dead, in the middle of a forest in winter.
A child who has seen that wants to know what happens next. A child who has been asked whether they have ever wondered something has been handed homework.
Give the explanation a middle
The most common structural failure is having no second act. Surprise on page one, explanation on page two, and then six pages of elaboration that go nowhere.
What fills the middle is evidence — the process of finding out, staged as discovery. What did people notice first? What did they try? What did not work? A child following an investigation is doing something active. A child receiving a finished fact is doing nothing.
This works even when nobody in your story is a scientist. The middle can simply be looking more closely: what happens to the frog over the following weeks, what is happening inside it, what happens when spring comes. The reader is being led through the mechanism one observation at a time, which is what understanding actually feels like.
Return to where you started
End on the opening image, changed. The frog in the ice, now seen for what it really was. This is the single most satisfying structure available to an explanatory book, and it is almost mechanical to apply: whatever surprising thing you opened on, come back to it on the last page and let the reader see it differently.
It also does something no summary can. It shows the reader that they now understand something they did not understand eight pages ago, without telling them so.
On being accurate at this age
You will have to simplify, and simplifying is not the same as being wrong. The test is whether the simplified version would need to be unlearned later. "The frog's body makes a kind of antifreeze" is simplified and true. "The frog falls asleep" is simplified and wrong, and a nine-year-old will eventually have to throw it away.
When you cannot avoid a gap, leave a door open rather than closing one. "Scientists are still working out exactly how" is a perfectly good sentence in a children's book, and for the right child it is the most exciting sentence on the page.