I couldn't say if a Cooper's Hawk will take puppies or not, but general consensus here (and on wiki) was that a fox was unlikely to take neighborhood pets.
The summer after I'd asked the question, the fox family (now four) that had taken residence near my parents' home a few years ago were seen to have taken a neighborhood cat, had confronted a full sized golden retriever... plus the human doing the walking... and were in the process of mugging my neighbor's dog (chained up) when my father and neighbor intervened to save it. The dog was unhurt, but it was very, very lucky all the same.
If there's any doubt in your mind that a Cooper's Hawk could someday take a puppy, take steps to protect the puppy before you have to find out empirically.
"Words that make questions may not be questions at all."
- Neil deGrasse Tyson, answering loaded question in ten words or less
at a 2010 talk MCed by Stephen Colbert.