dd & ds don't want to see their dad because of his gf!

[mothergoingmad]

Hi all this is a bit of a long story!
My dd is 9yrs and ds 6yrs. I have been separated from their dad for 5yrs now and we are both in relationships with other people. ex moved into his gf's house and now has kids every other weekend at gf house and they stay overnight.
The last few months have been a complete battle as they keep telling me they don't want to go to dad's. They tell me gf is the reason, she went all out in the beginning to befriend them and now she really can't be bothered with them she has no kids of her own. she doesn't want to do anything with them and is downright rude to them. we had a big debate 2 weeks ago when ex came to collect kids and they told him didn't want to go.all reasons came out and he was very standoffish with them said if they don't want to go so be it his life will go on, when dd challenged this comment with "you don't care about us at all!" he denied it saying it was their decision but not to say he don't care. He is a complete arsehole with an attitude problem and personally I can't stand the air he breathes but I have spent the last 6mths at least forcing them to spend alternate weekends with him in an attempt to keep them having a relationship with their dad. I have come to the point where I can't do it any more. my dd has been in tears tonight as she don't want to go tomorrow. When I discussed it with her she said she's not sure what to do as if she says she don't want to go anymore then in a few weeks/months wants to see him she is scared of him then rejecting her and her left regretting her decision.
Has anyone been through this one and can offer any advice on how to deal? I do not have a civil relationship with ex we only speak when we have to.
Sorry warned you it was a long one bit I've had a gut full of this it is driving me insane!!!