The mice might appreciate it. It would save them from traveling far and wide to collect the Numbered.

Nel has noticed the change herself, as she only sees Russell intermittently. Nevertheless, the same logic would apply to most adults regardless of how thin they may or may not be.

His hesitation is noted, and she shakes her head, “You don’t have to if you don’t want to. I bet we can find a bigger entrance if we look far enough. We can look where most people don’t go.” And thus, where no one would have thought to put a grate.