Maud, please read the thread on What to consider if you are buying a puppy. Val and I will repeat this till we are blue in the face - buying a rescue dog is often NOT an unknown quantity, as many breed rescues take in dogs whose history is well known, and are only being rehomed because of non doggy reasons such as divorce, emigration, illness etc. Most reputable rescues will also foster dogs before they are homed to ensure dogs are OK with kids, pets etc. and will offer lifetime backup, behavioural support, a network of other adopters, and so on.
Yes, many rescues don't adopt out to families with very small children but this is for very good reasons - any dog requires a good deal of time, attention and exercise, and it also needs to be able to spend its time without being mauled/prodded and poked by enthusiastic toddlers. If you're not willing to put the commitment in on this, then wait until your children are a little bit older before taking on a dog. With two tiny children, are you sure you can commit to at least an hour every day for exercise, especially as your DH is not around much of the time? Are you going to leave DC in hte house while you walk the dog, or will you take the entire family on a daily expedition?
Some rescues will home to families with small children, but they will want to be sure you have thought about the issues above very carefully for the sake of your children and of the dog. Generally, it takes a great deal of commitment and experience to make it work, especially if you do end up with a pup and such young children.
Also, not good to think of family pet as being a guard dog. Some breeds rarely bark, and many family pets are more likely to lick you to death. 