More than likely she has chosen a quiet and safe place for the nursery, and
there will be no reason for her to move them out in the open where they can
be seen.  Unless the humans assume a proactive approach and seek the kittens
out so they can be handled, by the time they venture out on their own they
could well be difficult to catch and/or tame.

The downside of that is: once they are found and handled, she may move them
to a more difficult spot.  It's a tough call to make.

That has happened to me in the past, and I had to resort to a crab net (hoop
net on a long stick) to chase the little buggers down so I could tame them
and eventually get them fixed.

If you do this, make an effort to also catch the dame and get her spayed.
There is no sane reason to allow an outside cat (male or female) to breed
twice a year.

In seven years I have caught, fixed, and inoculated more than 50 cats who
casually traipsed through my yard.  Some were adults and some were
kittens--35 of which were females.

--Geno<up to his *a.s*terisk in cats>Royer