@denim321, I've been through this, it's really not.
Let's say OP has bought tickets to go to the theatre say, with her DH in evening, their first night out in months, so their DC have gone to OP's parents for the night. EXDW rings at lunchtime on the day of the event and says to the DH, would you look after DC this evening as I want to go for a drink with a friend?
Response A. For the DH to prioritise not upsetting his DW is to say to his EXDW is I can't this evening, as I've already got plans, but I can pick DC up tomorrow (or whenever).
Response B. For the DH to prioritise not upsetting the EXDW is to say sure I'll collect them at 6pm. Meaning he'll have to cancel the theatre plans, waste the money and put his first date night out with DW off, so his EXDW can go out instead.
Neither of these responses have anything to do with the DC, and aren't a reflection of the DC being first/not being first. It's everything to do with who the DH feels will be less upset with him, and who the DH feels has more "power" over him.
My EXDP would have gone with response A every time. I (and our joint DC), were at the end of a very long queue which put DSS first in line, swiftly followed up by DSS' DM.