I remember reading a blog post. It was about current US players (College mostly), and the question someone asked them was "what was your least favorite moment in sports?"
The response that came out the most was.... "the ride home with my parents"
Because this meant adults questioning kid's actions, actions of other players, coach's decisions concerning tactics or playing time.
I think a better approach when a child enters the car after a game would be to ask "What did you learn today?" and let them do the talking, so they are the ones carrying the conversation. Another strategy I have used is saying something along the lines of "Can I share some feedback with you?".
If the answer is Yes, then means that there's a door open... but allow the kids the chance of saying "no" or "not right now" What are some of your feedback strategies? Feel free to share!