How do you handle a child laughing when the other team keeps losing?

I’d deal with it quickly and calmly, then move on. I probably would not stop the whole game unless the other children were genuinely distressed. At the next natural pause I’d get close to him and say something simple: “You can be happy when you score, but we do not laugh at the other team.” He may genuinely not understand the effect it is having. Then look for the next positive moment and reinforce that instead. If he celebrates with his teammates and runs back ready to play, great. I would avoid labelling him as unkind in front of everybody. At six, this is exactly the sort of social boundary they are still learning. If it keeps happening after clear reminders, then I’d speak to him more privately between games and involve the parent if needed.