Gotcha. I think that a well-coordinated mass of 5-year olds could bring someone down to their knees pretty quickly. I know that you're not allowed to touch a wall, but if I were the big guy here I'd have to stand with my back to it to have a chance.
Which brings us to the natural question, gonores (if you're still around), what happens if you DO touch a wall?
ooh, ooh, I'll ask and answer this one . . . each wall is covered with nasty pointy spikes, ala the final room in The Running Man. That'll make sure you don't back up against it, and it will make you wary of putting your back to it as well!
Seriously, I think the Magic Number is 10 or 20 less than normal (at least) if it's very difficult to stop them from getting behind you.
Another consider: offense/defense. Do they automatically attack? Must you automatically attack? The attacker, it seems, has the disadvantage here. Is there a time limit, i.e., you have to bring them down in ten minutes? Fifteen minutes? An hour?