Help! feral cat has 3 kittens under my porch

[althe]

Feral cat has been populating the neighborhood for nearly 2 years and she had 3 kittens under my porch who are 1 month old tomorrow (about 18 days old in photo). She used to bolt at first sight of me but now stays in the box when I leave the food. The cardboard box is now too small for their size + mother. Shouldn't I be putting them in a bigger box. She's very protective and has refused to eat the 3 or 4 times that I've examined her babies. Who knows how she'll react to me changing the box. I don't think I'd approach while she's there, I don't trust her not to attack.

How do I proceed to try and save this family? It crossed my mind to try and trap her and then to transfer the family to a room in my basement. This way I'd be fairly sure to save the kittens but I don't honestly know if this is actually a viable option. If she decides to move them, I'd likely lose sight of them which would result in three more unsterilized feral cats in an already overpopulated neighborhood.

I don't have much real life experience with a situation like this. I live in east end Montreal and the pound is a well known high kill place (Berger Blanc) and I don't know if there are, or how to contact, any TNR (catch neuter release) in my area or even if they could help. Advice of members will be greatly appreciated.

Thanks,
Teresa