So very glad it helps, Calisa. I think scenes where the backstory is especially painful are the hardest to write--glad you worked through it.

Now I agree with using snippets and dialogue to tuck in any back details that are necessary to move the present action forward. I did this in my first story sold to Cricket where the mother died and that was why the teen was watching her brother while the Dad worked. Then some details as to how the teen knew certain knowledge to better the present situation and get the kids out of trouble. Thanks so much for this, Angela!

It's hard to answer this without knowing just how important the story is on the current story, but my advice is to really challenge yourself as to whether the backstory is needed. If you take it out, are the story stakes no longer clear? Is the character's motivation uncertain?

You can include it at any word count, but any backstory you include must FURTHER the story in some way, ADD something important that ties into the current scene (context) and be something where it can not be delivered in another way (such as symbolism, etc.) The shorter the story, the more I would look into what you could do through dialogue and symbolism, and whether alluding to something (such as a past trauma or event) is all you need, rather than showing it more fully.

With children's magazine fiction, you especially don't want to pull readers away from the current story (as would happen if you used flashbacks). So if you include some, make it a snippet, such as a detail that suggests something important, or symbolizes a reason behind a behavior, an emotion, etc.