Age of Characters

I think "children's stories" is different to YA, so I doubt you're in any trouble with your genre there.

And character ages in winning stories have run the gamut from children all the way through to the elderly. By and large, your goal is just to make sure your character is interesting. Age isn't important if the character is compelling--and it certainly won't save you if they aren't.

I’ve found that MG runs a fairly wide age range which requires different levels of complexity. For example, the Mercy Watson series are silly books about a pig who is treated like a child and her misadventures. Pictures are essential to the storyline, the plots are uncomplicated, and the sentence structures are simple and repetitive. These are clearly children’s stories that wouldn’t do well in WotF. On the older end of the spectrum are books like Harry Potter, the likes of which would absolutely make good WotF submissions because they appeal to a wider audience. The characters face truly difficult problems, the worlds and plots are complex and engaging, and the solutions are unique. Both series are considered MG.

I think that as long as you have a well-written story, you can absolutely write about a younger protagonist. The winner you mentioned, “The Squid is My Brother,” was an absolutely brilliant story about an elementary student facing all of the problems of being the new kid plus the problems of being alien. I loved that story.

I've read many a story in recent volumes with adult protagonists. Also quite a few with teenagers or younger as the protagonist. As Jest stated, it was how compelling these characters were that made the story shine, not so much their age. 
I'm sure your story about YA or Middle School kids is fine.

@cweaver Children’s stories is a publishing category mainly defining the age of readership, just as Middle Grade, Young Adult, and Adult are categories that publishers use to try to define who will read a particular type of novel or story. You can have Adult categorized stories about teens and preteens—just look at Lord of the Flies. The theme and subject matter was aimed by the author at a mature audience, but did scoop up me as a teen because it was about boys and about kids my age. I’m sure some of the deeper symbolism shot past me. Still, there’s no question that book would be categorized by publishers as Adult.

Just because you write about a six-year-old child does not make it a Children’s story. Depends on target audience, depth, and subject matter. Just be sure MG on up would find that story relevant and engaging, because the publisher here will only select MG on up—they have stated their target audience. Incidentally, my winner was about a twelve-year-old disabled girl and her robodog on the Moon. Yet, even though it was about a tween, which might be classified as MG, “Super-Duper Moongirl and the Amazing Moon Dawdler” brought me well over a hundred comments and emails from adults that loved the story. Heh, I recently got an email from the band leader of the international heavy metal band Cloven Hoof, telling me how the story had moved him and made him cry, and that he would now buy anything I write. 

The age of your protagonist will have some impact on the category publishers will try to place your story in, but what will determine it is subject matter, treatment, and the age level your coding is at. Make sure it hits what the publisher requests in their guidelines…if you hope to sell to them. ?

@wulfmoon Thank you, Moon! Your story sounds great! The MG books I've written were targeted for kids between 10-13, who definitely enjoyed them. Adults enjoyed them, too, but not as much as the kids did. I'll remember who my audience is, and use a bit bigger vocabulary when appropriate -- and I promise, no fart jokes!

Nothing wrong with writing about younger characters if it's appropriate to the story. Sounds like you're on the right path. If you're uncertain how to proceed, reading some of the most recent volumes of the anthology is a decent way to get a feel for the kinds of stories the contest is looking for. Doing critique exchanges might be another good way to figure out if you're aiming for the right target audience (although it's important to keep in mind that not every person you trade with will necessarily be your ideal reader, and you should view any critiques you get through the lens of whether or not they improve the story you're trying to tell--there will always be times when people have a different idea of what the story "should be" than you do, because they would have written it differently).