When in doubt, use positive terms when speaking to children.
One, negative descriptive words (e.g., bad, stupid, lazy) are applied by the subconscious to the self, not to a relevant behavior. And it is that much worse when such words are directed at the self. Further, the subconscious does not differentiate whether the parent is being serious, silly or sarcastic; it interprets the word and any associated emotions, not the ‘intent’.
Two, the subconscious ignores qualifiers such as ‘do not’. The child may refrain from a proscribed action, but the subconscious reads the communication as a negative expectation, i.e., “If an adult had not been here I would have done that action. Since the action is bad, I must be a bad person. If I was a good person there would be no need for the adult to stop me from doing a bad thing.”
Speaking positively seems like a no-brainer, which is exactly why it is so hard for parents to revise their language. They speak fears learned from their own parents, which are perpetuated by their own insecurity and negative self-worth.
Yet speaking positively has a dual benefit. As parents learn new ways to communicate with their children, the children obviously benefit; but the parents reinforce their own worthiness as well.