how to lure kittens from their underground nest?

I am new in this group and last week for the first time in my life I rescued
a cat using a trap. She/he was an app. 16-week old kitten and used to live
around the graduate apartment complex of Lehigh
University,PA. She/he is already in a no-kill shelter. This week I was
supposed to trap the mum and bring her to the same place. But I found out
that mum gave birth again. The kittens have their eyes open and they walk
but their legs are still weak. Fortunately, the shelter agreed to take them
all. The problem is that I can't get the kittens! Their nest is a ground
den, probably confiscated from a
groundhog. Unless they come out, there is no way for me to grab them from
their nest. I can't even see them when they go deep inside the den. And they
only come out, when their mummy calls for them. Does
anyone know of a method on how to lure kittens?

I cannot afford to wait until they get older as the university told me that
they will trap the cat(mum) themselves if I don't manage to do so soon. Just
now, they don't know about the kittens. I'm afraid
that they will contact animal control, have the mum put asleep. Is it legal
to euthanize a nursing cat in the US? If yes, is it still legal when you
can't locate the kittens?

Thanks for your help
Ozdesh