WOW! First off a huge congratulations!!! What a wonderful thing to do.

It is so hard to know how the kitties will react. I'm sure there will be some stress in the beginning and the ones you least worry about may be your biggest problems. You know how that goes!
I really don't know how you can prepare them other than if you have friends with small children, you could have them visit often or even babysit and see how the kitties do.
I do have one suggestion that may impress the social worker and that would be too get the kitties used to wearing SoftPaws. That would eliminate the fear of a baby being scratched by one of the furry boogers! It would also give the kitties time to get used to wearing them.
Sorry but those are the only suggestions I have. When the time comes for the baby to arrive you will just have to deal with each problem as it arises, and maybe there won't be any!