I share 2 children with my ex.
My ex has a new wife and 2 new babies.
He has not been able to control me now for almost a year (court ordered). He has now, a month before christmas tried to claim universal credit for 4 children which in turn has stopped my benefits (I also work 37.5hrs) but have rent of £1.2k. I explained to him the 2 child cap. He laughed when I said I could loose my house and stability for our children. I feel so stuck. No one believes how vindictive he is. Before court he had them on his days off. Now, he makes his wife look after them all week and only sees them a grand total of 8 awake hours the week he has them. He doesn't care about them he only cares about hurting me and I'm not sure how much more I can take.
If your children live with you as their primary care giver and residence then UC is only claimable by you, they cannot stop your claim, that is simply wrong and in breach of their rules and guidance. He can only claim benefits for the two children that live with him.