Labradors are active, fun-loving, energetic dogs that, when under the influence of excellent dog training, can be a great family pet. Known for being good with children, it is not a natural tendency for any dog, even a Lab, to automatically be kind to small children. Introducing your dog to small children is a process that may take a little patience and time in a controlled environment where you can protect both your child and your Labrador from getting hurt.
One of the major issues that is important to maintain for Labrador health, and a successful introduction to children, is to make sure that your Lab gets plenty of exercise. Be particularly sure that she runs and plays and expends energy before you begin the process of introducing her to your child. The context for the mutual introduction should be an environment that is calm, not a time for play, and preferably indoors.
Have your Lab sit or lay down and allow your child to calmly approach her. Show your child how to gently pet your Lab. A few strokes on the head will likely delight the child as well as 
instinctively demonstrate boundaries and dominance in the mind of your Labrador.
If your dog seems appreciative and licks your child’s hand, or wags his tail, encourage the interaction to continue for a bit. If the dog lifts his lips in a snarl, growls, seems annoyed or disturbed, or attempts to escape, let your Labrador have her space, and save introductions for next time.
On the next occasion, discourage your Labrador’s negative behavior with a firm “No” and encourage better behavior by giving her a reward after letting your child pet her. If your dog continues to have a defensive posture, don’t press the issue immediately, and don’t reward her.
Acclimation can be slow. Never just assume your Lab is safe or leave her alone with small children.
Good post. If a dog is well socialized right from it’s puppy-hood, then the dog should be good with children too. Socializing should start at an early age. And for training, Positive reinforcement training(clicker training) is the most effective and humane way to train dogs. Dominance theory and aversive methods are debunked methods.