Letting the little kids win sometimes is kind. Teaching them to cheat to win is not.

Easing off so a small child gets a result is one of the nicer things older siblings do, and it has an obvious end point. Showing them how to bend the rules is a different lesson and has no end point, because what they learned is a method rather than a feeling. One gives them a good afternoon, the other gives them a habit.

Keep the generosity in your own play rather than in the rules: slow down, give them the first move, miss one. When they break a rule to win, say so lightly and carry on. What you are teaching is that winning has a shape.