Sadly no there are no written stories from me about them. And they don't appear in the dark future story because the plot takes place right after Twilight became a princess in the show, it would have to take place a few years in the future for the kids to be in it. The only thing I have so far is scattered illustrations with small descriptions of moments in their lives and there's more in their reference sheets, other than that I'm afraid I have nothing. And there's a few more in my tumblr page, like this one hazurasinner.tumblr.com/post/1… that shows how the mane six managed to have children.