I'm not sure people realize how their words can adversely affect others, especially a child. The vulnerable little minds of chilren can be either negatively or positively impacted by what we say to them. Positive words can encourage and help children thrive into adulthood. Negative words can sit there for the rest of their lives, influencing and affecting how they see themselves and their world, and ultimately how their life plays out. Some children are strong enough to rise above the onslaught of negative talk, but others will just succumb to it. Tell a child he's stupid and, more often than not, that's what he becomes. Tell a child he can succeed and, more likely than not, he will.
I grew up during a time when 'reverse pyschology' was de rigeur. It worked for me because I was a willful, stubborn child (and young adult) and I wanted to prove people wrong. I remember my mother used to tell me I would never graduate from college, my university counselor told me the same thing- so to prove them wrong, I graduated from college- with honors. But for some, those words might have prompted them to just give up.
Build up children by encouraging them, telling them how clever and talented they are, how they can accomplish anything they set their minds to. Tell them how much they are loved and appreciated. You will be doing them a great service.