Sometimes kids act like they're okay with it, bit in this case it was not good from the beginning.... so why did it get this far, with him knowing his kids never liked you?
Is it that they have become more expressive since your marriage.
Maybe they thought it wouldn't last and seeing you getting married, they realise it's not a casual relationship.
The eldest being vulnerable with you is a good sign. If he hated you that much, he wouldn't live with you...unless his mum is a worst alternative.
You say his kids shouldn't have a say over your relationship...but if it's making them so unhappy that can't be ignored at this age they are. I wouldn't marry someone that my kids hated... that was a big mistake..what did he think would change?
Do they say why they don't like you?
Is there a large age gap between you and him?
Children from separated homes can go through a lot of emotions, which the parents are unaware of.
Do their parents get on well? Was it an acrimonious split?
Did you meet soon after he split from his Ex?
You don't have to answer, but all these things make a difference to DC.
Is it just because you aren't their mum?