It sounds like they are mostly concerned about the welfare of the kids. Is it possible that they are thinking of whether or not this issue makes you unfit as parents and may want to challenge your custody of them? (in that case, you could say they never had a problem with it before they know, so how does knowing this suddenly change how well you parent?) You might want to read Redpepper's blog or PM her. Her parents thought that their grandson's home situation was abusive and I believe they took them to court, but now they all get along very well when they proved that her son was not being abused by anyone.