Janeway-
While reading your post I had the exact same thoughts that Karen expressed - if these cats are letting you get close enough to them that you know they are sweet, they are probably "dumped" as opposed to "feral" cats. Especially if they are "hanging around". Truly feral cats are rarely seen - you only know they are around because you hear them, or catch them as they are eating your cat's food, and even then you usually only see their backsides as they run for their lives. So it may not be as difficult to tame them down as it may seem.

There are alternatives to cages, although they may not be available. Keep in mind that the kittens will have to nurse for 6-8 weeks, so it will be that long before any spaying can even be done. Add this up to two month before they give birth (the gestational period is about 9 weeks - if they are visibly pregnant they are probably somewhere beyond their fifth week by now). A spare closet, bathroom, basement, etc. would be ideal, but I realize it's hard to find someone willing to devote a portion of their home to the exclusive use of a cat. The only cages large enough to be in use for the extended time you will need are very expensive - a minimum of $100. But even a feral cat knows how to use a litter box - it's in their nature to cover their waste, so they will naturally use any sandy substance for their elimination needs. And in an emergency, a full-sized box is not necessary - I've used those cheap aluminum foil-type baking pans they sell in the grocery stores for about 99 cents.

If it were me, I would rather wait until the current litter is born. That seems cruel, but I can't imagine those cats, and then their kittens, being caged up for weeks. If no one is able or willing to take them into their homes, I think they would be better off being left alone for now. It would still be possible to trap and tame the kittens, although it may not be possible to catch them all by that time. There would also be less of a chance that all the kittens would survive. But the first rule of cat rescue is - remember, you can't save them all.

I hope this helps, and God bless you for wanting to do something.