The child who changes the game may be experimenting with a better way to include someone.

A child rewriting the rules mid-game is easily read as cheating, but the change often has a purpose behind it. Someone smaller cannot reach, someone new keeps losing, someone is about to be left out entirely. Children negotiate belonging through rules because rules are the tool they have been given. What looks like breaking the game is sometimes an attempt to repair it.

Before stepping in, ask why the rule changed. If the answer turns out to be about including someone, say that out loud so the others hear it counted as a good idea rather than a broken one.