Most of the times a family already has a dog or a puppy and this situation arises when they have a new baby into the family. As many couples start their married life having a dog with them and after a while they think about having a baby when the dog has already established itself in the family.
Another instance would be when a child is adopted into the family or the dog is adopted in the family already having kids mostly because children wanted a dog. In any case it is important that u make sure this mixing process goes smooth and safe.
Lets start with when a new baby arrives in family with dog. In this case it is important to start the process even before the baby arrives. So the key is the preparation u make. You can prepare the dog for the arrival of a baby by making it used to the baby stuff such as baby's crib, clothes, bed, toys. Even get your dog used to the sound of baby by playing it over recorder and make dog listen to it. Let the dog know where the nursery room is, and it is a room where you go into and close the door and dog has to stay out of it. Make sure you pay enough attention to the dog when you are not in nursery o that the dog understands that what you do in the nursery does not detract from the attention that the dog receives.
With young dogs make sure that you provide him obedience training so that the dog responds to your commands and learns to follow what you ask. With safety as the prime concern a obedient dog is more likely to provide safe environment for a baby. And when baby grows n starts crawling and walking the obedience training proves important. Because not all dogs feel comfortable with baby around and they look at baby either as a threat or a prey. Always praise the dog for being polite with the baby and never leave the dog alone with baby. Always keep a watch over dogs behaviour and reward him for good behaviour. With time and encouragement your dog will get used to the baby.
With slightly grown up children it is important to teach them how to play with dog gently and respect dogs food and water dishes. The dog behaviour varies according to different breeds. Some breeds are more tolerant than other, so consult your vet about the exact points to work with for your specific breed.
Never leave your child alone with dog until your have taught the child how to be nice and respectful with dog. Not all dogs tolerate if their tail is pulled or somebody grabbing their fur. So teach your child how to pet the dog without hurting it. Never allow the child to hit or poke into dogs eyes. Threatened or startled dog may get aggressive growling and even biting.
If you consider the above points and work over them then the mixing of dogs and children should go safe without any mishaps.