A lot of soccer parents with good intentions give a 30 minute lecture, covering all the players supposed deficiencies and giving playing advice, in the car on the way to each match. The kids arrive far off their optimal mental state, and dreading the critique they are likely to hear, whether they want it or not, on the way home. Kids who are massaged in this way tend not to play badly, they just tend to not play, possibly to avoid making mistakes.
The easiest way to detect this problem is just to ask the player if it is a problem. Kids are more than willing to share this grief. The easiest way to correct this problem is to speak to the parents, as a group, about your expectations, and to cover this as a routine problem. Many of the parents will recognize themselves if you can present this problem with humor and illustrate the importance of the kids having fun and arriving in a good state of mind.
For best results, parents should memorize and use the following
Before the Match
I love you
After the Match
I love you
It was great to see you play
What would you like to eat?
Source: Bruce Brownlee - USSF "C" License - Tophat Soccer Club - Atlanta, GA