T.T.
I would personally let her bring the baby. As a former single parent we all need a leg up and it sometimes truly takes a village.
I wouldn't agree to it until you renegotiated your pay rate tho.
you pay her for a certain amount of care for YOUR children in YOUR home.
If she wants to invite other children other than yours into your home, there should be a payment for that (ie; discount on your current rates for bringing the baby)
I would talk to the Nanny as well. Let her know your concerns and fears.
I do however agree with others who have said they couldn't bring their kids to work, etc. But these aren't easy ok times anymore. And sometimes compromises have to be made for the greater good.
How blessed could your children be by having a new baby in the house. Better yet, think of the blessing you are going to be to that baby, who would probably get substandard care from someone else, and for that single mom, who probably doesn't have any other way to get on her feet.
Good luck to you.