Tell them that they are being ridiculous (warning this may be the cause of more tears but dont worry it gets better) Pull them a side and tell them that they are amazing and that they will win the next match

If your kid is tougher then tell them that everyone will think that you are a baby because you are crying because you got beaten.

Tell them if they play baseball,football,hockey,wrestling that the very next play
they will win the next play or football ull get the tackle block the guy whatever
then tell them too just brush it off and make the play

You must be sensitive to each different childs needs for example my son would cry only if he knew he should have beat the kid and others would cry just for losing. I like to tell them that if they feel they did all they could do to win and fell short than we must need to try something different. I do NOT like to say oh it's o.k. because it is obviously not alright with the child. I have been coaching youth sports for almost 14 years and have had plenty of cryers, liars and high flyers and the one constant is that nobody wants to lose and sometimes they just need a hug and some affirmation.