Start with "What is a story?" -- a story tells us about a series of events that happen, and can be told verbally, visually, or in writing. The goal of a story is usually to entertain (books/movies), educate (such as fairy tales that have a lesson), or remember (history).
The very basic structure of a story is that all stories have a beginning, middle, and an end. The beginning tells us how the hero (protagonist) is in their "normal" world -- whether it be modern day earth or a magical world like Narnia or Middle Earth. In the beginning we learn who the hero is, what they're life is like, what kind of flaws do they have that need to be fixed in the story? (Like are they greedy? Are they poor? Are they selfish or mean?)
Then ask the students to think of their favorite movie or book and tell you how it starts and what the hero is like in the beginning. Ask them if the hero has any bad traits? Like Tony Stark in Iron Man/Avengers starts out selfish. Or Captain America is stubborn. After a few discussions from kid's choices, see if anyone is still struggling -- if they are, ask them to name 4-5 movies they love and work with the class to see if together you can find the beginning and ask those questions.
Then have all the kids create a character of their own. They need a name, a few details like job or hobbies, and they need to know what their characters flaw is that needs to be fixed in the story.
At the end of the beginning the hero runs into a new problem -- this is what the story is about and this is called conflict. Every story has conflict. Go with the kids and give a few examples of problems famous movie heroes have -- Tony Stark is kidnapped. Frodo gets Gandalf's letter and has to leave the Shire. Whatever this "problem" is, it disrupts the hero's normal world so they can't just ignore it and go about their lives. They have to face this problem.
Go through with the kids and make sure they all know what their hero's problem is. If they struggle, as about the hero -- where do they work? What are they like? -- and help the kids come up with a problem that fits.
That brings us to the Middle. The middle of a story is where the "fun" is -- the hero tries to defeat the problem is a number of different ways but usually fails. If he/she doesn't, the victory is minor and doesn't last. There are ups, and downs, the hero wins and loses, maybe they go through a training session where they gain powers (Iron man testing his suits), maybe they make new friends (Frodo and the fellowship), maybe they fight some villains. The key is this is where the hero does any learning they need to do. This is where the hero makes any friends they're going to make. This is where the hero tests him or herself and prepares to face the big bad guy in the ultimate showdown.
Work with your kids and have them each think of two or three moments that will happen in their middle. These moments should be: making a friend, learning a lesson, facing a bad guy, or discovering something that will help them in the end. The moments can be anything, but give some examples from famous movies or books to help give them ideas.
This brings us to the Ending. The ending is where the hero has to face the bad guy and it's time for the hero to win or lose for good. This is where everything is decided, and the hero fights the bad guy one last time and it's the biggest fight of the whole story. Have your kids think about their Ending -- where will the fight happen? Who will the hero be fighting? How will it end?
Once they finish, they have the basic outline of a story. There are a lot of details to be filled in, a lot of wording -- but that's the frame of a general story. You can explain to them that the flaw we talked about in the beginning is usually something that the hero overcomes throughout the story and it's something that the hero learns from. You can explain that the story will be made up of a lot of little moments of ups and downs where the hero will win, or lose, but the key is that they all connect to the main goal -- whatever that problem was that came up at the end of the Beginning. Everything the hero does is about him or her trying to achieve that goal.
If you want, you could go through this as a class and create a story of your own. I'd recommend writing some down beforehand that you can easily tweak to student suggestions like AdLibs style. Maybe take some children's books that are very short to show how this is seen in them. Or some comics even since they're visual. I think the three act structure - beginning, middle, end -- is the easiest breakdown for that age level as most understand that, and if you get too involved in the variations it can cause confusion until they have the basic structure down.