In addition to the often recommended “I love to watch you play!”, some suggestions of supportive words to offer and questions to ask the young player that will help them grow as a teammate and in their understanding of the game:
You worked so hard today.
I saw when you helped that opposing player up, that made me smile.
I am sorry that today was rough. I’m so glad you have tomorrow to try again.
It was fun to see you have fun with your friends.
What did you do today on the field that made you feel proud?