She will probably bring up D7s bday party soon or to one up will actually organize one herself before you can start thinking and discuss this with the kids. It is okay to let her know that as long as OM is involved you are in no way trying to keep her, let alone win her back. She is no prize at this point in time. How much do your Ds understand about the OM? They are still young but D7 may have some idea at her own level.
