A high speed, great volume wag is a happy tail. But a high speed low volume wag of the tip of the tail is coming from a very tense dog. A low speed, large volume cat-like swish of the tail signals a dog to be avoided at all costs.
Similar cues can have very different meanings.
This is true of most canine facial expressions, body signals, and noises. Sometimes identical cues have contradictory meanings, depending on the context and whether the dog is interacting with a person or another dog. It is difficult enough for us as adults to reliably sort out all these signals coming from our dogs. The key to mixing dogs with children is Safety First. And the first rule of safety is awareness.