Hello Sara.
You state that you are looking for a watchdog, but do not say anything about what size dog you are looking for. As an aside, any dog can and will bark and keep watch. From a security standpoint, the size of the dog is not really relevant; most people think twice when they hear a dog bark. Regardless though, you do still need to take responsibility for your safety. Just the presence of a dog won't keep you safe if you make yourself an easy target.
As far as breeds and children, while I am reticent to say anything about any breed for fear of offending anyone, because someone will have had a good experience and a bad experience with any breed, I will say this: the dogs bred for guarding or protecting in any capacity, have higher bite records. This includes German Shephards, Pinchers, Rotweilers, Border and other Collies, and other herding breeds, Chihuauas and Poodles. Part of their protective tendency is to control other animals and that includes breeding for a tendency to bite or nip. With children that is something to consider.
I myself have 4 border collies. I love them to death, and think they have been wonderful with my child and his nephew (my son is not yet 3, my nephew is 5 months). They are mid size dogs, sleep near the boys, watch over them, and take treats gently from my son, and always have. It's been a huge part of the training investment I've put into them. Mine have never bitten anyone. However, my ex husband has one as well, and twice she's bitten my son. Just something to consider.
When you make your decision, in addition to your needs for protection and good for children, as has been suggested, take into consideration what you can offer in the way of the animal's care for your own ease. Will the animal be indoors or out, or both? Do you have a large grassy backyard, or small one? How close are your neighbors (some breeds are more inclined to bark, and may get you into nusiance barking troubles)? Do you have carpet, or floors and furniture that are easier to clean, for shedding purposes? How much time do you have to devote to a fully coated animal? Check out the price for food for the animal-- a larger animal will eat more and cost more in food. Your son is very small and young yet. It will be a while before he can manage any size dog, but at his size, a big dog stepping on him or knocking him down is going to happen. How much dog can you handle? Check out training programs from a kennel or dog trainer (I recommend you avoid places like Petsmart-- someone who trains dogs for a living can help taylor training for your needs), and plan to attend some classes with your dog. What kind of energy level can you manage? If you can't get out and burn that energy off the animal, you're going to have other issues emerge as a result.
I grew up with Cocker Spaniels. Never had trouble with them. We grew up in the deep South, so when it got hot, to control shed, they were groomed into a shave. But I have a scar on my jaw from where a neighbor's Cocker attacked me completely unprovoked as I walked by as a 5 year old. We also had Dobermans. Never had trouble with them, but I took one of ours into another area several times when a friend with her baby visited because each time the baby cried, the dog began to get upset and focused on my neighbor. I can only assume because she was "making" that baby cry. My parents now have Labradors. They're great dogs. But they are large, and high energy. They've run my son down and knocked him down unknowingly many times.
For the sake of animals in general, I would recommend you go to a shelter and have a look around. Ask to spend some time with a dog that interests you in an enclosed pen. Take your husband and son, and see how the animal reacts to you, your child and a man (one of mine is a rescue, and is afraid of men). If you can, go back more than once, and at different times of the day. If you adopt a rescued or shelter animal, you save a life, and you may be able to arrange a trial period if problems emerge once you have the animal in your care.
I wish you good luck. A dog can be a wonderful addtion to your family, if you chose wisely and take responsibility for it.
K.