You should be able to get a restraining order, preventing him from seeing your children or contacting them or any member of your family in any way. A good solicitor will have this in hand and sorted quite quickly. My daughter's husband was not any way as bad as your ex. and she got one very quickly indeed. Stand up for yourself and your children and don't let him anywhere near you or your children. You may need to change solicitors if yours isn't being forceful enough on your behalf. If need be, move away and go into a women's refuge with the children. Let the police know you are there so everything is above board. Just don't take any chances, please. Believe me, moving away with your kids is not a matter of money, but a matter of determination to keep everyone safe. You can find a way to do this and rebuild your lives in a safe environment. Just hang in there and make sure your voice is heard. My thoughts are with you and I really do wish you all the best.
Just an idea : Make an appointment to meet your ex somewhere he will have to drive to. Arrange for the police to be there when he arrives, driving his car. They will probably be delighted to have the chance to arrest him ... yet again! Just make sure there is no way of him finding out that you dropped him in it. The police need to make the arrest on route to where you are to meet, but not at the same place. Sooner or later he will go to prison, if he keeps re-offending.