Warning signals are body signals and vocal sounds a dog will provide to create social distance. In plain English the dog is asking for "more space".
Here are some posters with warning signals:
The dog's mouth is slightly open (stress), ears are binned back and whale eyes are great indicators that the dog is not enjoying the interaction. In this case you should remove the child and provide the dog the space and peace it is seeking.
The black Lab is threatened. She is baring her front row of her teeth, her muzzle is frowned, her ears are pinned back and she growled and later lunged and barked at the "thing" that stressed her out. Don't forget that the dog is stressed. Not happy when they are providing these signals. Respect the signals and give them what they need. Distance!
With the help of a trainer you can work on desensitizing your dog to sounds, touches and other things that stress them out. Never use corrections. You really want to tackle the underlying emotions that is causing the dogs to be afraid and upset about something.