My 4 year old son and I have been considering getting a dog lately. Breed help?

We would obviously need something good with small kids, although my son is big and very mature for his age. I have a medium sized house so I*m thinking a medium sized dog. I would also like something a little more on the calm side, I hate jumpy snappy dogs (and yes I understand I will have to train the dog not to do these things, I just feel like it would be easier with a calmer breed). Maybe a little bit on the watch-dog side because my son and I live by ourselves, but not in a scary way (if that makes sense?). Anything you have to throw out there would help because it*s hard to look up every single dog breed and read all about them!! Thanks! :)