A.H.
Dear F., My daughter is only 18 months so I have not had to face that issue yet. One thing that you might consider is talking to your middle daughter every couple of days, say before bed or another special time between the two of you, and tell her something about her dad... like how much you and he both love her, how he misses her too, etc. It doesn't have to be a lot at all as her cognitive level doesn't require it. You might also give her a picture of herself and her dad to put in her room. While I understand that you may not want to see his face any more than necessary, which I completely understand, it might help. You can tell her that whenever she misses him or wishes she could tell him something she can look at that photo or talk to it like it was him. A little strange maybe... but a six year old's world is not very rational to begin with... they can have imaginary friends, right? Maybe your ex will call her more, but in the meantime, good luck!