This might include a large sun umbrellas, sunscreen, and a hat with brim to shield your face and ears. A blanket in the winter can be a big help. Turn off your cell telephone as a courtesy to your child and all the other players. Do not talk directly about the match for at least one-hour. This allows a cool down time for both of you.
Parents can say hurtful things because they are so wound up in the match.
Your child has just come off a very competitive event, so let your child chill-out. It's too late to explain how they should have played the match and where they made their mistakes. If you are a tennis player, you should note that you were not the one on the court and that your child had to play it the way they saw it at the time, based on any number of factors. It is a lesson learned if, after the match, you are able to analyze the match and determine what you might have done differently, if you had been the one playing. Or what your child might need to work on to be a better player and win the next time they come up against that type of player.