EDIT: I’ve included a link to the original post for context.
Well, I’m not “literally” standing up because I was sitting when I wrote that post (and am sitting now as I answer this). But I am literally supporting them. (I’m one of them!)
Just because you focused on the point of children finding this content (which is justified, I suppose), I decided to do an experiment: I went to image search on Google, turned on SafeSearch, and just put in the names of the main six characters: Twilight Sparkle, Rainbow Dash, Pinkie Pie, Rarity, Applejack, and Fluttershy. The most sexually suggestive images I could find in the first few pages were Twilight Sparkle in bed and a back-nude humanized Fluttershy, which aren’t explicit. Down the list are some “wet-mane” and “bedroom-eye” ponies. You can try this for yourself to draw your own conclusions, but I think people who are into this (actually Google and SafeSearch, really) indeed have done a generally good job of keeping this away from children.
I also want to bring up a post I reblogged a few months ago. It specifically concerns the blog “Ask Princess Molestia” and the “Down with Molestia” movement against it, but it makes a good point about the whole “children might find it” argument. In short, it’s the parents’ responsibility to make sure that children aren’t exposed to things they shouldn’t be seeing. The post states that content creators have given parents the tools to do so (“ratings and tags”). I think I’ve taken the appropriate measures on my end. My blog description advises that this blog is “highly NSFW.” The blog has been marked so on Tumblr since I created it, meaning that my posts are generally hidden from people on Tumblr unless they disable “Safe Mode”. (More information.)
I am concerned about young children being exposed to this sort of content. But the Internet has much more than this that children shouldn’t be seeing, including content based on other cartoon characters children enjoy. This, too, has been kept out of “public” view, and My Little Pony is no different. (Just for reference I searched Princess Peach, which as I’m writing this is at the top of this list, which is basically a “most gotten-off-to” characters list. I didn’t find anything inappropriate for children.)