And that very child they mistreated turned out to be very successful in life, while the child they played favoritism with turned out to let them down over and over again!
Does that parent go back and apologize to the mistreated child for doing them wrong? Can they say to that child "I am so proud of you" with words or by making eye contact? One thing I've learned in life, you just don't know what's going to motivate a person.
Remember, it's never the child's fault. They didn't ask to come into the world, but because of a decision you made they are here. So stop blaming them and stop taking it out on the child.
Remember, you just don't know. You might need their help someday.