What is 'the afternoon?'
Apologies if I've missed this but there is a big difference between a 2-4 pm slot and 12-6 pm. What is 'the afternoon' to one person can easily feel like nearly a full day to another.
You also mentioned that they've helped you out in the nights before. What sort of time commitment did this involve?
I think what I am finding difficult in judging their response is it sounds like they have provided assistance in the past but it's not clear to me how much and whether that evolved over time to perhaps a much bigger commitment than they were anticipating.
Regardless of the past or how much time you are thinking of now, I suspect this could be part of their reservation - if they agree to this, where will things go from there? Is there an end point? Are you able to say - I need this help for the next six months then I have x solution which doesn't involve you.
There are certainly some grandparents happy to do on-going childcare but others aren't for whatever reason, reasonable or unreasonable. They may well feel like they are giving you a blank cheque.
If you've told them your ex won't help as way of encouraging them to help you, this may have actually backfired as a strategy. They may be thinking - if WE do this the ex DH may feel he can check out even more, as we'll always be there.
I really strongly believe that it is not as simple as prioritising their social life with the church. These are two unrelated things tbh and I wouldn't also raise that with them either, sort of suggesting they are only entitled to outside interests after they've sorted out your needs.
I am sorry if this sounds harsh but I think to get the outcome you are hoping here you need to be honest with yourself and ask how you are coming across to them.