I would guess she has the kittens in the hole - but this is just a guess. Mommy cats can squeeze into tiny areas with their litters. Watch her closely. If she suspects someone has found her kitten location, she will move them: one at a time, to a new place.
Not only do you need to be aware of dogs, but also of un-neutered male cats. They are probably more of a threat. An intact male cat will kill a litter of kittens which then brings the mommy cat back into a "heat" so he can breed with her. It may even be one of the fathers of the kittens (there can be more than one male who is responsible for a litter).
I had a sad situation when I was 18. A community female had her kittens in a neighbor's shed and a stray tom we took in (and neutered, but he was older when neutered) would kill a kitten one-by-one and leave them by our back door as a gift to the family and maybe to "show off". I managed to save one kitten and the mommy. I kept the kitten and got mom a new home.
Watch her closely. The area she is protecting is most likely the location. If you see her going from one place to another repeatedly, she is moving her babies.
Good luck! I hope you manage to find a safe haven for the mom and her little ones. Keep us posted.