Interesting problem. If you can get the authorities to grant you just one
more week, you should be able to trap with some assurance of getting
everyone in a livetrap, as the kittens will be exploring on their own
soon, whether momcat approves or not.

There are some pretty specialized livetraps that would help, as well.
There is even one that is made to fit over a hole in the ground or fit
flush to an opening in a wall or embankment. And there just may be one
designed to fit into a den opening. Just be sure it is a livetrap and not
one that would kill or inflict injury.

If you have the opportunity when momcat is away, try luring the little
ones out into your hand by reaching in and making loud! meowing noises
(sounds silly, but it works). This works well if the kittens are 3-4
weeks, when they are toddling and innocent. I once grabbed a whole litter
by reaching into the den as far as I could and mewing like a kitten with
its tail in a wringer. The kittens all toddled right into my hand. Then
as we were leaving, I called one more time and the last kitten answered!
Got him, too. And how did I find them? By mewing like a kitten with its
tail in a wringer.

It is a fine line, trapping a family with small kittens when you can't get
to the den and don't know how many kittens there are. You can probably
easily catch mom, but then the kittens are left unprotected. Or you can
caught in the act by the momcat, who then moves the litter to parts
unknown. Or you can catch some but not all of the kittens... and so on

I guess in your case, I would first plead for more time and then I would
trap mom and get all the kittens I could lure into my hand, hoping to gawd
the den isn't so deep they won't be likely to toddle out.

Good luck. And keep us posted.

My Campus Cats address is below if you want to consult.

Sharon Talbert
Friends of Campus Cats