I know how to play chess and I can teach my child. Why do I need to hire a coach?
A coach is an outside person. S/he will provide more objective view of your child's chess playing skill. As human being, we, as parents, are always looking our children's games in an emotional way, either too rosy or too harsh.
A coach has more experience to pinpoint your child's weakness so s/he can work right on the point that needs training.
A coach can help you save time and money by pointing you to a more effective route based on what s/he has learned from coaching numerous other kids.
A coach has more resource and training material that can be offered to your child.
A coach knows more about tournaments and clubs.
A coach's words are much more higher valued by your child than parents' sentences, so his/her suggestions are more likely to be followed.
A coach can help plan your child's chess career with better success chance.