P.P.
I would highly recommend labs for families. Make sure you research the breed BEFORE you buy it. Some dogs are just terrible with families with young children so, be prepared to do some research. We purchased our dog from a breeder 11 years ago and our three children were young then. Pure bred Labs and Golden retrievers have wonderful personalities with children. Granted, large dogs require a lot of exercise, (if you don't have access to walking the dog or a fenced yard, don't buy a large dog, it just isn't fair to the animal)
and care of their coats.. they do shed but you would not have to worry about them being aggressive at all.
It would be nice to adopt a dog from a shelter. You just have to find out as much as you can about the animal's past. And you take a gamble on its background. But the advantages are they definitely are way more affordable and you are helping those animals find a wonderful home. Keep in mind that YOU will be doing most of the care for the animal if your children are really young so make sure you are ready for this responsibility as large dogs live upwards to 12-13 years of age. My children have wonderful memories of the dog, we all love him, its just dogs are a big responsibility. Cats pretty much take care of themselves!
good luck!!!