I have written exactly two CYOA magazine stories for ages 8-12, so far. Both of them have sold. I thought you might be interested in how to write one.
Starting the story is fun! Let's make up something...say, an avalanche. OK. Start at a peak of action. I usually like to yell. So our beginning might go something like this:
"Look out!" You yell and duck into a cave on the side of Avalanche Mountain, as snow plummets from an overhead cliff.
Remember, this is just an example. Were it my real story, I'd come back later and play with the wording. I suggest reading it aloud while trying to think from a kid's viewpoint. If you have kids near this age, you might ask them to read it back to you. Make sure you use as few words as possible to give your readers all the information they need to imagine and understand what's happening. And, make sure it is easy to read aloud.
Now, you have to split the story into two or three directions (yes, this soon), and give your readers a choice:
You are trapped in the cave. "Sam! Sam!" You call when you can't find your brother. You wonder if Sam is safe or if he is burried in snow. To your way out, go to #7. To look for another tunnel leading out of the cave, go to #12.
I don't actually assign numbers until after I finish the story, but when I do go back and add numbers, I try to vary them, so the reader feels like it's a bit more active, like a scavenger hunt. Next, you continue the story, but now you are writing two entirely different stories with the same beginning. Let's say the reader turned to the paragraph numbered 7. It might read like this:
You start digging through the snow covering the cave entrance. You yell for Sam and think you may hear a weak voice, but after five minutes you are sure you must have imagined the voice. Your hands are so cold they start to burn. Just then, you remember the battery operated lamp stored in your backpack. To keep digging, go to #5. To search for another way out with your lamp, go to #2.
Do you see how this works? I like to print my story and tape it on a poster board or wall to see how it flows. You can draw connection lines on poster board.
Both of my stories were from 1700-1800 words with about 20 choice numbers. Each story had four endings: one super end, one pretty good end, one OK but not great end, and one tragic end.
For a super-steller story that an editor REALLY wants, write the story around factual, historical events or objects so the reader learns something. The publisher will likely include the historical info somewhere after the story.