"Although I certainly understand why you would like to do that, based on the comfort level of the other parents, I am afraid that the request is not something I can accomodate. I am sure you understand, as you would likely not appreciate having someone unfamiliar around your own child for any extended length of time".
I would let her know that she's more than welcome to come by for a coffee visit, but that outside of that, you also have the routines of the other children to take into consideration. Make sure she knows that for the first little while, until her son (HER, really) settles in, she is welcome to pop in as long as it's not something like lunch or nap time.