The first point to make is that the majority of players feel that they play better in practice than they do in matches. So, if you feel that your child is not playing as well in a match as they can do, then bear in mind that their opponent may not be either!
If it is your child that feels they should not be losing, reinforce to them that lots of results don’t reflect rankings or ratings.
You could show them a drawsheet from a tournament showing several examples of lesser-ranked players defeating higher-ranked opponents.
There are, however, things that both parent and player can do to help bring out the best tennis in important matches.