They should be avoided. Dogs who are afraid try to appear smaller by crouching down low. Their head, ears, and tail will be down. Their mouths will be tense and closed. If you approach a dog showing these signs, he might become aggressive out of fear and he should be avoided.
Friendly dogs are relaxed.
Usually their tails are wagging in greeting and are below the line of the back but not between their legs. Learn how to read a dog's body language to help you determine if he's friendly or not. Children should always hold out their hand close to their body, flat fingered with palm facing in when meeting a new dog they want to pet. If the dog shows any sign that he does not want to be petted, do not allow the child to pet the dog. If the dog backs up, do not allow the child to pursue him.