Support every single child and encourage them to do better. Your kids will follow the example you set.
One of the most important lessons that a parent can teach a child is to respect others. Not everybody on a team gets along and can be friends, but the number one thing that any person must have for another person is respect. Parents should be a leading example of this at all times.
Not every child on a football team, or really any youth sports team, is a superstar. Not every player can throw the ball accurately, run fifty yards for a touchdown, snap the ball cleanly or be a shutdown cornerback. In fact, it takes a lot for some children to even have the courage to step on the field. So, there are inevitably going to be situations where children make mistakes on the field.