Explain everything and warn in advance what is waiting for the child.
The unknown is always frightening. Even kids can easily explain what they will do, avoiding "scary" words. For example, they will not make a "prick", but "inflate the ball" or "freeze" the tooth. The doctor does not" drill", but" vacuumes", or" expels the worm", which is started in the mouth and bites the teeth. Here you can let your imagination run wild. Rest assured, the doctor will always be happy to support your story.