For children who lack self-confidence, it's not wise to undermine their self-esteem further by emphasizing their mistakes. This will make them more withdrawn and afraid to try harder.
Some children are spoiled brats, they are the one who need people to remind them not to have bloated head and big ego. Other children have high self-esteem and are motivated to prove people's beliefs are wrong about them. Then they are more suitable to be taught that they are a piece of $h*t, their high self-esteem make them work their way up to disprove the negative opinion.
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