Ah! Now you are adding further information - in other words, you are now telling me it is NOT just that he pays no child support - in fact he has assaulted the children AND they do not want to go. Those are reasons that WOULD be relevant to a court application to reduce or stop his contact with the children.
If there are no court orders in place concerning his contact with the children, then you can just stop his cotnact with the children - but you must write to him to inform him why - his violence and the children's wishes. But do NOT say in your letter that it's because he is not paying any child support. If you stop his contact, he might decide to apply to court for a contact order to get it reinstated, and he might include a copy of your letter with his application to court. If you say in your letter to him that it's because he doesn't pay any child support, you will immediately lose the sympathy of the court.
If there is already a court order for contact, then after you stopped his contact and written to him, YOU must immediately apply to court to get the contact order changed.
Whichever one of you applies to court, the court will consider the position of both of you, and decide what contact if any the children should have with their father based on what is best for them, not which parent shouts loudest.
As I said above, it is government policy that children should have contact with the parent that they do not live with - unless there is VERY good reason indeed why that should not happen. But the welfare of the children is paramount, so if they are at risk of being assaulted in their father's care, then that is very serious, and could mean that the court orders that all contact should stop - or the the court might order that the contact could continue but must be supervised eg at a contact centre.
With regard to the children's wishes, the older they are, the more weight the court will give to their wishes - so if they are 12, 13,14 or upwards, their views will be taken very seriously indeed by the court, and it's unlikely that they will be forced to see their father if they don't want to.
It might help you to get some face-to-face legal advice from a specialist family law solicitor - here's where to find one:-
I hope this helps and I wish you the best of luck.
Thanks and best wishes...