Play a short game where winning is mostly luck — snap, a dice race, matching pairs.
Play to win rather than letting them. When you win, say out loud what losing feels like and what you do with it: 'I wanted to win. I feel disappointed. Shall we play again?'
When they lose, name the feeling before offering anything else. Do not rush to a rematch until the feeling has been acknowledged.
Losing well is learned, not innate, and it is learned by watching an adult lose out loud. Naming disappointment and continuing anyway models the whole sequence: the feeling is allowed, and it does not have to end the game.