Jodie, your children, while small now, won't be small forever. One thing you need to take into consideration is your family's overall lifestyle. Are you athletic? Couch potatoes? Do you (and I'm assuming) and your husband work full-time? Are the kids in school? If you work and the kids are in school, there will be no one in the house for hours. Will you hire a petsitter, or try to crate-train your dog, or take him/her to a doggie day care? Or will you leave him/her outside?
Will you commit to training your dog? How will you handle things if your dog gets separation anxiety? What if the dog chews the furniture (or shoes) or pulls on the leash? Will you walk your dog regularly?
I ask these questions because I've had several dogs. They all have differing challenges and rewards. Information is your friend. Here are a few resources to get you started:
American Kennel Club:
http://www.akc.org/breeds/index.cfm This is so you can get an idea of what breeds are out there. Even if you get a mutt, that mutt is composed of two or more breeds and there are interesting things that go along with each breed/group. For example, here's the link about Boston Terriers (the dog in my avatar):
http://www.akc.org/breeds/boston_terrier/ All the way at the bottom it includes information on how to shop for that particular breed. If you get a purebred dog, please get it from a reputable breeder. That's someone who will take the dog back if things don't work out, and will help with socializing and who will take pains to assure that genetic diseases aren't passed on to your pooch.
Purebreed rescue:
http://www.akc.org/breeds/rescue.cfm If you don't mind an older dog (and they are often trained and more mellow than a puppy, which may turn out to be ideal for your family), please consider going through a purebred rescue outfit, as these groups are as committed to protecting the health and temperament of the breed as breeders are.
Of course there are plenty of mixed breed dogs out there. Please consider getting one from a shelter. Try these links to find a shelter near you:
www.MSPCA.org or
www.hua.org You can save an animal from almost certain death and get a lifelong family member in the process. You will have less information on temperament so ask a lot of questions of the shelter staff before taking anyone home. I've had both good and bad shelter experiences. Our best dog, Jake, came from a shelter.
Finally, unless you're going to show a purebreed dog, please consider getting him/her spayed or neutered. It is better for the dog's health, more convenient for you and is just plain more responsible.