There have been much emphasis on speaking positive and encouraging words to children, or anybody, rather than negative ones. Our words are either blessing or curse. Yet, some may over do it by ignoring the need to be truthful, and just say positive words that paint a false picture. Especially when the facts are, sort of, "negative". For example, what should we say to a kid who had scored badly, say 10%, in a test? ... What would you say?
Should we tell him that he hadn't done badly, and should be thankful that he still managed to scored 10%? Is this better than telling him how stupid/lazy he is that he failed badly and scored only 10%? The former is "positive" but to say he hadn't done badly is falsehood. The latter is obviously negative, but rightly state that he failed badly.
In a post a few years back, I mentioned about an article I read that stresses on the importance of consistency in raising up children. Once we started speaking words that are not true, children will notice our inconsistence, and eventually treat our words as unimportant.
Another article I read more recently talks about when parents praise their kids, they need to do it in alignment with the facts. When parents over praise their kids, like the above "positive" statement, kids will be less willing to strive harder for better results.
I think the first pre-requisite in speaking is to be truthful. Then, we choose to speak it in a positive way. In the above example, we should let the child know he has failed badly in the test. Try to substantially work with him on how can we, together, make it better next time. When a plan has been worked out, we can positively emphasize that we appreciate his willingness to want to improve. Thus, there is no falsehood, there is encouragement, and there isn't condemnation.
This is not only about kids. In general, being truthful is so much more important than speaking positively.