There is nothing worse than seeing your child trying hard and playing well yet being denied a victory by an opponent's bad line calling and/or gamesmanship. It is also very difficult to know what to do during a match if you think it is happening and what to say at the end of the match to a distraught child.
Remember, one bad call, in your eyes, does not constitute cheating - anyone can make a mistake, especially in the heat of a match. However, if you see a pattern appearing you should contact the referee immediately and express your concern.
Unfortunately, there’s not an awful lot else you can do. It is something that all players have to learn how to deal with and the best way to learn is probably to fight it for themselves.
Immediately after the match is not the time to discuss what your child could or should have done. Support, understanding and a shoulder to cry on are the main things your child needs right then.
Leave the discussion and the advice until later when things have settled down. The thing that will amaze you is how quickly they get over it.