My older girls get along beautifully with our dog but my son is another story.
My pre-schooler loves to play with our puppy, but lately the dog started barking at him. I held her snout and told her, "no." However I'm concerned that her behavior may escalate into something more serious.
After reading a few articles, I don't think I have sufficiently addressed the situation. Most experts seem to say that you have to make the dog stop the behavior until you are sure that the dog has backed down.
If you simply say, "no" and the dog only stops momentarily, you may be reinforcing the idea that the dog is in charge. There are a number of suggestions for modifying behavior, like spraying the dog with a bit of water, or using a clicker.
After the dog backs down, experts suggest leading the animal to their crate or bed. Often times, dogs will stop the behavior but first need to be taught what you actually want them to do.
It's also important to talk with your child about playing properly and learning to lead the dog.