Polar fleece blanket will be excellent! They are gorgeous!
You will probably need to keep them for at least 6 weeks, if possible its better to keep them until 8-10 weeks (breeders won't sell before 10 weeks) until they can have their first vaccine. I don't know if the mother cats remember lost babies, but it is common to lose a one in a litter (I've seen it happen with both cats and dogs)
Birth truly is a beautiful thing isn't it!
Wishing you all the best with your little litter!