This, to me, is appropriate for adults as well as most children. Of course, my kids grew up watching movies like 'CARRIE' and 'The Exorcist', so maybe I'm not one whose opinions should be used as parameters for such things? Of course, in our culture today, I've witnessed kids as young as three or four say some pretty shocking things.
I love dogs. I have four myself, and you certainly have got their quirky behaviour flaws down pat. That, you CAN be sure of!
I really enjoyed the idea behind this, I'd love to see the pictures you have to go with each line. I'm assuming each line is a page in a picture book? Are you going to use photographs of your own dog, or illustrations? I think photographs would give it a nicer, more personal touch, but illustrations are classic, and would still have that personal touch if you did them yourself.
My only concern is just the content. It deals with realities people have everyday with their pets, but I'm still not sure parents are going to willfully buy their children a book that states, "Your dog is going to go to the bathroom, and you're going to have to clean it up." And I know flat-out there are swaths of the country that won't touch the book just because of the whole leg-humping thing. Yes, dogs do it, but that doesn't mean people want to buy a book that highlights the fact. It's got the nice ending that says, despite this, you're still going to love your dog, very much, but... I'm just concerned that it will get relegated to a gag gift and not have the sincerity I believe you were going for.
Of course, if I ask myself if I would buy this book for my children, I find myself saying, "Yes, I would." But I would also buy my children books like "The Paper Bag Princess" and "Go The F**K to Sleep", so my opinion may not be the best one to follow in this regard.