I understand this issue completly. I have three children through domestic adoption. My 2 sons have the same birthmom. I speak with her on a regular basis, but since she is out of state, there will be limited opportunity to meet in person. The birhmom has been incarcerated a few times for drug use and is currently in rehab, she has no family support so calls me frequently asking for both emotional and financial support.
My oldest is only 3 1/2 so at this point he only identifies her as a family friend, eventually we will tell him the true relationship ( still trying to figure that discussion out !)
My plan is to try to do most of the talking myself and only put the kids on the phone once in awhile to say hi. I also have concerns that the birthmom may try to aks the kids for money when they get older ( wnohter issue I need to figure out).
You may want to consider reducing the number of visits and use phone rather than visit in person. YOu may also condider discussing their poor choices.
I have told my son that his birthmom ( i just use her first name with him) is in the hospital becasue she took some medicine that did not belong to her. We talk about her not making good choices and that no one should take medicine that is not given by a doctor.
I have another interesting dilema, we hear from my sons birthmom all of the time ( weekly) but NEVER hear from my daughters. I met her last summer and even brought my (then one year old) daughter to meet her ( I had to fly half way across the country to do it). She does not stay in contact becasue it is to painful and she is trying to move on. It will be difficult to explain this in the future.