I am an Indie author with several published books and I have done quite well (I sell several hundred books per month). I have books for MG and adult readers, but I have several children's story books I would like to pitch to an agent. Should I stress platform in my query since I have quite an established platform and would work very hard to promote my work, or will that even be a factor since it's such a different genre?Let's review what platform is (and thus what it is not.)
Platform is NOT published books (unless the published book has an excerpt from the new book.) Platform is how people will know you've published a new book.
So, if you're selling several hundred books per month (and that's a terrific number, congratulations) the way people find out about your book is platform.
Are you working a mailing list?
That's replicable so, yes, I would count that as platform.
Are you Tweeting and Facebooking to let readers know your book is out there?
That's replicable, so, yes, it counts as platform.
Is it being listed as a "deal of the day" in online bookstores?
That's NOT replicable (or at least it's outside your control unless you are self-pubbing) therefore it's not platform.
Are you gaming the system to get listed as a NYT Bestseller?
Well, people know about you, that's for sure, but probably not in the way that encourages further sales.
Platform is how people know you now, and how they will learn about your new book.
Now, to your actual question.
I'm not sure what you mean by a "story book." Generally books for young readers are
Those books are bought by parents and librarians, not the actual readers, and much of those sales are review driven.
It won't help your editor reach the picture book market to know that you've sold eight gazillion copies of A Really Good Novel.
The way to promote picture books, and other books for young readers are school visits, not Facebook or Twitter. Have you done a school visit? Do you know what they are? Remember, it's not some sort of failure if you don't. None of us were born knowing all this stuff. I still learn things every day (almost always the hard way of course.)
If you want to branch out into books for younger readers the very first thing to do is find your nearest chapter of SCBWI and join. And go to meetings and events even if you're not sure they're relevant. You need to soak up information before querying. Books for young readers are whole different ballgame from adult books.