The Omen: Good Omens
I was just rewatching the remake of The Omen, and I knew a lot of the first episode of Good Omens was a huge wink at the movie The Omen, but I didn't realize how much.
Just like The Omen, the father is an American diplomat in England. There is a hell hound at the kid's birthday party, and, of course, he gets a weird nanny who is there to guide him toward evil.
The comparison ends there, though, The Omen is a serious horror film, and Good Omens is an awesome comedy.
In any case, I strongly suggest you watch both.