The way you lose a game in front of little kids teaches them more than any speech you’ll give.

Children watch behaviour and ignore instruction. A talk about sportsmanship is noise; what registers is what your face did when the whistle went and your team had lost. They file it as the normal way adults handle losing, and they will produce it themselves in about four years without knowing where it came from.

Pick the next loss with a younger audience present and do it deliberately well. Shake hands, say one decent thing, keep your voice level. Then let yourself be properly annoyed about it later, out of their sight, because that is allowed too.