Any tips for the usual mystery on how to get seat assignments on Turkish when you've booked your tickets via United (016) which includes Turkish segments?

Turkish says you have to call United to do it. United says you have to call Turkish. HUCA over and over with Turkish but no luck.

We are 4 people on 3 different reservations, so we won't automatically be seated together on check in day.

I have had this problem on Star Alliance carriers forever, completely futile on getting seats when booked via United. Being *G doesn't help at all. Since I often book the kids on miles tix and pay for my own, it's hard to get us sitting together... Any tips?