What kind of a game? A sport or a fitness game? Serious competitive game or an entry to the sport game?
Younger participants get a boost from being paid attention to in a positive manner during a game. A good play followed with praise, a muff followed with a “hang in there” or “good recovery”. Never single out a player on a bad play, give encouragement for errors anonymously.
Older children need a bit more reality. If there is time and a place, give instant positive instruction. If it was baseball ground ball missed scoop, “That grounder got away from you. We should probably spend some time on them.” Then if there is a spot off out of the way, “come over here and let me try to get some by you”. If not, a pat on the shoulder and “Keep on going after those ground balls. We can work on them next practice”.
End every game with a bit of praise. Look each player in the eye while saying things like, “good hussle”, “good focus”, etc. Try to never show too much negativity or disappointment. If you stay positive, the children will be more likely to stay positive. Be warm and upbeat at games and practices. Wins are easy. Losing efforts are “learning experiences”. Everyone can have an off game and losing a game is a chance to improve for next time.
I don’t know if this what you were looking for, but I am not afraid to miss the point. Nothing ventured, nothing gained.
If the children see you being enthusiastic and excited about a game, then most likely they will feel that way, too. If you can keep it simple with a game where everyone is a winner and does their best–versus a competitive game where some may lose–that may help with them leaving with a positive memory.
Have fun with it!