The issue here though is that children rarely see the relationships in the same way the adults do.
So the man had an affair and is now living with someone else and they have a new baby on the way etc etc. This has turned the OP’s life upside down but he is still their father. They will develop their own independent relationship with the OW and with their new sibling. And as much as adults often want to think that the children will think as they do,so very often they don’t,and that can go either way.
I know a setup where the woman had the affair and left for OM. She moved with him to Spain, left the dd behind with her dad and the DD ended up living with him full time.
But fast forward several years and the DD spends the majority of her time at her mum’s with her, the new bloke and with her siblings born of the new relationship, even though the dad was the one who raised her, was there through the aftermath and one might think would have gained the most loyalty from the DD.
Children don’t think as we do. Sometimes a split does turn their world upside down, but sometimes it doesn’t in the ways we imagine.
The OP hates the new partner for the part she played in the breakdown of the relationship. But the children may not. They may in fact love their new sibling. They may look forward to the time spent with the dad and the new partner. It’s simplistic to suggest that if someone is a bad husband they are a bad father. They may not be, however much we want to think that’s the case.
The elf is a minor detail here. What needs to happen is that the OP needs to support the DDs’ relationship with their father going forward. As for her DS, you only have to look at the stepparenting boards to see that most women wouldn’t seek to maintain a relationship with DSC should their relationship with their partners break down. he is under no obligation to this child, however much we’d like to think that he is.