I know EXACTLY how you feel. You don't want to be too demanding because they will be mad but you don't want to be too submissive either. NPD to a tee.
The only thing that worked for me was the nice approach, I hate to say it. It practically kills me to have to beg for money.
I used to say,
You have always been so reliable when it comes to the money situation for the kids. When can I expect the check?
Short and sweet.
I got temporary support filed so I didn't have to deal with him. He still pulls this crap even with alimony but he knows I have the law behind me. They have to be held accountable, too. Even my XH thought he was above the judge. Most of them do. Feel as though they are victims. I am sure you know the drill.