Excellent job! I bet the owners are very pleased with it.
I was drawn to this because of the word "simple" in the title. I always make simple sites (my thinking partly influenced by seeing that it didn't do Google any harm). I have had quite a lot of sites (since 1998) but I've never once had anyone ask me if I could make them more complicated!
It's crying out for one thing, a "home" link. I clicked on the big header picture expecting to go the home page (pretty much a convention now) and discovered it wasn't linked. I would link it to the home page.
On the "FOR SALE" page there is just a contact email. I think it would be better if there was pictures of the dogs for sale, information about them, prices etc. I appreciate though, that this might come with time.
Looks to me like you certainly understand the purpose of a website, which should stand you in very good stead indeed.