Through a parent's eyes(and heart)I can empathize completely which is why I chose to operate a dc and stay home with my own children. As a dcp, no way could I have worked with a demanding parent, only because it's group care. If you cater to 1, you cater to all. And that just doesn't happen when you've got multiple children in your care.
She either 1) needs to trust you 2) get a nanny or 3) stay home until her children are older.
If you want to keep these dcks I suggest sitting her down or a heart-to-heart. I can only imagine how tough it is placing an infant's care into another's arms with complete trust.
Just wanted to add that maybe (if you already don't) you could snap pictures of her infant throughout the day, just for reassurance. Giving her that connection may help.