Congratulations on becoming a Foster Mommy! It is great fun...and awful giving them back.
It is a great adventure, fostering, but a lot of work, especially with a litter, and you need to be prepared for her to be somewhat skitz...liking you one second and maybe even growling and snarling the next. You CAN do this!
Mommy Lab Mix can go either way...nice as pie with humans and/or other animals around her babies...or evil queen of the universe. This is especially important since YOU will be included in her reaction and she does NOT know you.
If I was doing this and she was already my dog and knew me, I would handle her babies as early and as frequently as possible...I'd be there while she welped them.
However, with a dog that does not know me yet, I would personally go quite slow with her at first, ignoring the babies and focusing all my attention on making her Queen For A Day Every Day until she understands I mean no harm...that I am only there to help her. I would hand feed her, bring her water, milk, whatever...getting closer and closer to those babies gradually and watching her reaction and body language, brushing my hand against them and speaking softly to her the entire time, watching her every and slightest move and expression. If she showed any sign of aggression toward me, I would back off until I re-gain her trust...setting her food down outside her puppy pen and making her get up to get it or wait for me to come back and serve it to her...which I would do in about 10 minutes...after she has time to realize the 'uh-oh' of baring her teeth, etc., at me.
To be perfectly honest, I would probably separate her from my Yorkies and kitties unless I am sure that Mommy feels safe...if Yorkie and/or kitty in a kennel, or viewable through a gate at the doorway disturbs Mommy, then I would wait until those babies are older. I'd keep the gate up so she can see the Yorkies walking by...keep the kitties away completely if she shows any protectiveness over those puppies.
From experience, I believe that she will have NO PROBLEM leaving those itty bitty babies to do in your kitty or Yorkie if she feels kitty or Yorkie is threatening her babies...IF she is so inclined to be hostile and/protective at all.
The upside here is that after the 'new' wears off the puppies and they start to explore, she will likely get friendlier. She will get bored being stuck in there all day tending to their needs and start wanting a little 'me' time and all the while be getting used to your household, too. Just my experience.
Besides stray and feral pregnant dogs and cats, I have had two Shep/Lab mixes of my own oops'd into welping thanks to me not being the only only who could allow that oops to occur...not everyone else was as careful, obviously
, and I knew both of my fur babies before they had their babies. One Mommy was never hostile to me, but did not want my kids anywhere near her babies and growled at them, baring her teeth. I, of course, stepped in and stopped the aggression, but also kept the kids back, too. Safety works both ways.
One Mommy was exceedingly nervous with me touching her babies, and I would not allow anyone else anywhere near her because she was horrendously over protective until they were walking, and even then, before anyone else but me touched them, I put her in a separate, closed off location. She would growl and snarl and bare her teeth if she even 'heard' another human voice.
I am not trying to scare you. Like I said, you can do this, and it is a wonderful, absolutely necessary thing to do...bless you for caring. I am only telling you this because they can go from "bitch" to "BITCH!!!!" faster than I can type it.
I am awaiting breathlessly reading your adventures...this can be one of the single most rewarding things you will ever do!