Any dog I've ever bought has a "Return to breeder" clause in the contract. Luckily, I never had to return one, but I think that is what a responsible breeder would do. Is this new litter from the same mom?
I think you might have other issues besides the peeing. All dogs pee. Could it be your friendship is not what you thought? Is it that he peed on your kid? Well, stuff happens. I doubt if the dog plotted some revenge against your child, if, after all, he was tied to the door and unable to flee.