If you've never heard of flash fiction, I'll make it very simple for you.
A flash fiction story is a complete story told in less than 1000 words. Some people will stretch that out to 1500 or even 2000 words, but I think that's more of a short short story. (Think of "short stories" as starting at around 3000 words or so.) It's that simple.
Although most flash fiction stories are considered adult stories, I've been struck by the fact that the length is about the same as most children's stories. Furthermore, the best flash stories seem to have fairly simple grammar and are told very directly -- again, like a good children's story. The great thing about flash fiction is that it's so short that it's fairly easy to study and learn from.
Today I just want to give you a link to an article at the Children Come First site called 6 Steps to Writing Flash Fiction. It takes you through the process of writing a 493-word flash fiction story, from idea to completed story. It shows you how to flesh out the idea a bit at a time, much as you would approach a regular children's story.
Here's a thought I would add to what is said in that article, especially since the article is about writing flash fiction in general, not specifically for children: Although children probably wouldn't catch on to all the symbolism the writer used in this story, there's no reason that you can't put it in if it helps you write better stories!
The article itself isn't very long, so take a few moments to pop over and read it. You might be surprised at how much it will help your writing.