I have mixed views on this.
Parents with care (90% of whom are female) suffer disproportionately from the costs of raising children. It's not just food, clothes and clubs, it's things like housing with an additional bedroom(s) for the DC, therefore with associated utility costs and toiletries, etc. And that's before even considering things like childcare costs to enable that parent to work. A non-resident parent paying a couple of hundred isn't even scratching the surface and doesn't have the mental load of working it all out either. Plus they usually have the benefit of free childcare provided by the parent with care (based on arrangements of EOW and one weekday evening, which is still by far the most common arrangement).
At the very least CM should include 50% actual childcare costs IMO.
In this scenario I'd want to know if the DC are having their needs met, and to a standard that is reasonably similar to what they could expect if their parents were still together.
The question is not whether it's right if the OP cannot afford to pay into a pension while her XH does. The question is whether she cannot do so because of this. Is she subsidising his pension?
I'd want to know if she sacrificed her career to raise the DC when she was still with XH? Can she not pay into a pension because she's paying for childcare? Because if so, he's profiteering off her hard work.
If OTOH she could work more but doesn't want to, and if XH is prepared to pay for childcare and is already paying way above normal living costs, then he's not doing anything wrong.