The game you teach a child also teaches them what you do when they beat you.

Playing a game with a child teaches two lessons at once. There is the game, and then there is the demonstration of how a grown adult behaves when they lose to someone smaller, and the second one is absorbed much more thoroughly than the first. They are studying you more than the rules.

So let yourself be beaten and let them see how you take it. Not by performing graciousness, which children detect immediately, but by actually finding it good. What you are teaching is what losing costs a person, and the answer they take from you tends to be the one they carry into every contest afterwards.