Your 21-month-year old kid is face-level with most dogs and therefore will be perceived as a threat in the dog world. Can you wait until your son is older? Properly raising a puppy IS as much work as having another child.
Once again, I must remind people that nearly ALL dogs that come from the Humane Society and rescue shelters have originated from unregulated backyard breeders and puppy mills. So while it "feels good" to adopt these animals, remember that you're spinning the roulette wheel. Yes- you can get lucky with a good dog. But it's equally as likely that you'll adopt some expensive genetic defects and behavior problems. In my opinion, "adopting" dogs supports puppy mills.
Staff at those chain pet stores are not breed experts. Their staff work for a little more than minimum wage and usually have no experience on farms/kennels or have any specialized training on animals. On the other hand, breeders who are conscientious enough to register with the AKC are regulated and do have to go through a bit of training.
Research on www.akc.org. Notice, this is a dot-org that wants to educate, not a dot-com that wants to sell you something. I know you’ll end up buying from where you want. I’m just advising that buying from an AKC breeder usually costs the same as adopting from a shelter. A good breeder won’t even sell you a dog if they think your family isn’t ready for one.
A final beware: My gentle 5-year-old son was viciously bitten in the head (6 stitches) by a rescue dog (listed on PetFinder) who was supposedly "good with children" according to the pet store staffer holding the leash (I'm not allowed to say the name of the chain store due to the lawsuit). However, I love dogs as much as the next person, and continue to do so!
(EDITED ANSWER for Katherine R and others). It's sad in this economy when families have to give away their pets. I sympathize. If these families had bought from a breeder, the AKC sales contract would require the pet owner to contact the breeder to take the dog back and re-home it. Under no circumstances would an AKC breeder allow their dog to end up in a shelter. So obviously, no shelter dogs came from an AKC breeder. Instead shelter dogs all ORIGINATED from puppy mills and backyard breeders. Not judging. Just saying.