Video their performances for reviewing.
This is a pretty simple one. Having a video of the game (smart phones for the win!) that you can look back on provides your child a great way to realistically assess how they did. You can help point out the good things they did and help them understand where they could improve.
If you can’t video every game, then at least try to fill out analysis sheets, so it’s possible to see trends and highlight potential for improvement. For example, all goals were scored from set pieces, etc.