A noble effort smeared by a sloppy execution.
Let's start on a positive note: in a world so plagued as ours, even the mere existance of books written to instill in our children simple but decent morals (because joking aside, kids, drugs really ARE bad) is the slightest of comforts. However, even the noblest of intentions cannot excuse an effort as sloppy as this.
Now, naturally, children's books are hardly an appropriate canvas for waxing eloquent prose, and it's common knowledge that repitition is good to get information across to a growing brain. However, there's a fine line between this and repeating the same information, at times almost verbatim, in the same paragraph, over and over and OVER. And it's a difference that, for kids, can cross the line from informative to dull - and what kid is going to take heed of a moral from a book they don't like reading? Furthermore, while it's indeed true that kids often relate to anthromorphics better than people, context has to be taken into account - and, in this case, as has been mentioned, the lack of opposable thumbs where they are so fundamentally necessary is apt to leave kids much too confused to comprehend a moral - even such a misguidedly heavy-handed one.
The illustrations really don't help matters either - if Miss Gibson really thought that such drawings were going to get the attention of five and six year olds, she really needs to rethink a few things - I mean, aside from the fact that no kid is going to find such realistically-drawn horses very memorable, she didn't even colour the things. It seems highly unlikely that kids will remember these drawings - and by extension, this book - as much more than lines on paper for very long.
There's one extra little element that makes this all the sadder, though: in and of themselves, the illustrations (surreal subject matter aside, of course) are actually pretty good - in fact, with a bit more colour and a few less joints, they’d actually work pretty well in a book featuring horse characters aimed at children a few years older (though, uh, preferably written by someone else). I’d have read it.
Overall, though, Miss Gibson, if you feel compelled to fight to keep kids off drugs, by all means, continue; it’s one of the most noble of quests. Just...find a different way to do it.
Oh, and let’s end with an obvious quip: "Latawnya The Naughty Horse" sounds like something off a shock site.