Fashion is a fun thing and both adults and youngsters can feel better by having styles that flatter them. However, a lot of this is just trendy stuff that comes in one year and is out the next--until of course it returns in a few years. Kids don't know this and they feel bad about themselves when they think they are not in fashion.
Children in our modern culture are over exposed to things should not see at such an age. It corrupts their minds and poisons their thoughts with ideas and theories that should not be there, for example seeing skinny people can make you want to be skinnyy and this could provoke eating disorders.
Children's values stem from their parents. A child who values fashion will learn that behavior from a parent. Even in older children, who will be influenced by friends and peers, the value set given to the child through example and enforcement by the parents far outweigh the effects of fashion on children.