When the three kittens Fluff, Muff & Puff emerged from the bag that they drowned in, the bag remains behind them on the ground, then it disappears in the next shot.
The gatekeeper lets the little kittens pass without checking their names or cause of death. One, he did say their names, and two, it was already obvious how they died. Why would you think he said, "What some people won't do."?
Since Tom is the only cat asked to sign the forgiveness certificate, it's implied that none of the other cats ever tried to eat a mouse... which is highly unlikely.