Our players celebrate opposition mistakes too much

I would deal with the behaviour without trying to remove the competitiveness. Make the line clear: we can celebrate what we do; we do not celebrate another child being embarrassed. Then give them something positive to react to. Big praise for the teammate who pressed, recovered or defended well means the group still gets that competitive release. I would not give a long sportsmanship speech every time. A quick “we celebrate us, not their mistake” is enough in the moment. Children often copy the emotional tone around them, so adults matter too. If parents laugh at an opponent’s miskick, the players receive the message very quickly. Wanting to win is fine. Enjoying somebody else feeling bad is the bit I would challenge.