This is not true. Your dog is very aware of the fact that your son is not a dog. Growling is your dog's way of communicating "Please stop, I don't like what you're doing. If you continue, I'm going to bite you". She's probably giving you lots of body language signals before the growling, but they are easy to miss.
How does your son interact with her? I ask because toddlers are known to not be very gentle. They grab, pinch, and fall onto dogs all the time. In addition, they are just strange. They don't act like adults at all, and that is very off-putting to dogs.
When she growls, does she have food or treats? Is she in her special place? Does she have a spot where she can get away from him if she wants?
