I am captain of a soccer team and we have 2-3 girls that are very negative on the field to their other players. Often pointing out mistakes, flaws etc etc. It's really degrading and just kills us on the field. Both girls are in their teen years and both think they are themselves, the next Matilda. How do I tactfully tell them that we need to work as a team, and their comments are really not helping without them getting very defensive and it just all blowing up in my face?