There is a difference between telling a child that they are a disappointment and telling them that you are disappointed with something they have (not) done.
I find that telling a child that you are disappointed with something they have done while underlining that you are confident that they are capable to do better goes along the same way as saying you are proud of a child's accomplishment. It is normal for a child to understand positive reinforcement as well as negative results but these should be expressed calmly and in context.
Too many times have parents resorted to screaming, yelling, and even worse, hitting their children for wrongdoings. This accomplishes nothing but the instilling of fear in your child and teaching him that violence is an acceptable means of resolving conflict. Most children have an innate desire to please their parents. They feel genuinely badly about letting their parents down. On the other hand, parents should not use this phrase to berate or belittle their children. We should not kick a man when he is down, however, to air our disappointment and talk about possible ways to correct the situation is appropriate.
Showing disappointment in your children is an appropriate way to discourage bad behavior. A recent generation seems to have no consequences to their actions. If the beginning of a simple phrase shakes a kid up for life, then their are more deeply seeded problems with that child to begin with.
Sometimes when a kid is messing up, the only way to shame them to do better is saying that phrase. I know it worked on me. This has always been my biggest fear when I was in jr high school was disappointing mommy and daddy. It's a very shaming phrase. I get frightened to hear it sometimes that I want to do well all the time. Disappointing my family and friends is the biggest fear.
But it's not abuse, because as everyone knows love is not a feeling it's an action. You can be disappointed and still love your children. What weirdo bizarro person would think saying that phrase is child abuse? You're not saying you're ashamed of the person. WTF?
Maybe I would say "I'm upset that you did this" but not disappointing. That makes the children feel like failures. Telling them you are upset they did that makes them feel like they did something bad but they can do better. Trust me I'm a teenager I know this feeling.
As a child/teen/young adult/ I was told several times by my parents that they are disappointed in me. I'm 35 now. I can still recall where my mother was standing each time she said those words to me and the anguish in her voice. Like any type of abuse, those things never really go away. In a child's eyes (no matter the age) when a parent says "I'm disappointed in you" what the child hears is "you are a failure." Self esteem starts to whittle and problems accelerate. My grandmother passed away two years ago and it broke my heart more than anything I've ever experienced. It broke my heart so much because I had lost the one person on this Earth who had never doubted me, was never "disappointed" in me. She saw the good and me and never doubted me. I knew without a shadow of a doubt her love was truly unconditional and I would never experience love like that again. Child need to be built up. They need to believe they are good. They need to know they make people proud. Children do not live to hear that they are a disappointment.