Grandma secretly helps our youngest win board games. How do we keep game night fun for both cousins?

Your son accepted the stated shorter route. That suggests his complaint is about a changing agreement rather than refusing to include a younger player. Acknowledge the difference before asking him to be generous: he already agreed to one adjustment.

Grandma's version of “not competitive” still picked a winner. You could say privately, “If we want to play without a score, let's choose that before we start.” She can help your niece understand a turn without secretly changing where a token lands.

There is a small absurdity here: the most determined competitor appears to be the adult who says winning doesn't matter. Keep any joke between adults, though. Your niece was playing the game she could see and hasn't done anything dishonest.