feel like we are starting over

Greetings to all you wise patient and loyal grands this is my first entry i'm new to the site and as we speak our 2 GD's 3 and 4 yrs of age are on their way to live with us. They are being escorted by their social worker and both parents rights have been terminated by the courts. I am totally excited can't wait to pour into them all our love and nurture but heres my point of contention.
The BM has already begun to call my wife on a regular basis asking about the kids and if we have talked to them recently. She will continue to do this i am sure once the kids arrive. When they interact with her in the past i have seen them totally change, act out and it takes quite a while to calm them down. We want to establish a routine with the girls and get them settled into a good positive pattern without interruption.
My question for any of you that have gone through this is, should we allow the kids to talk to BM on the phone right away even if it seems to cause change in their attitudes, how long should we wait before allowing conversation and what if it never gets better, what if we always get negative reactions from the girls after they talk to BM?
I been praying about this one i just want to approach it the right way, so give us some of that good wisdom out there as we start over raising kids.
thanks
Rev.DECĀ