Mooly:
Whatever you decide to do make sure you have a certificate of parental responsibilty. Without it you only have a financial obligtion to the children and your (ex?)partner can make it hugely difficult and distressing to see your kids.
If you decide to call it in then also make sure you have an access agreement in writing that is signed by both of you. One drawn up by a solicitor will obviously hold more clout but having something in wriyting will make it that much easier to maintain access should it get hideous.
If you leave then contact the CSA yourself and find out from them what you need to pay and make arrangements to do so. If she says she doesnt want to go through the CSA then get this in writing. Any financial agreements you come to get in writing.
Above all and no matter what do not get abusive, rude, offensive etc despite whatever provocation may come your way and how hard it gets. Always concentrate fully on maintaining the relationship with your children above all else.
From experience walking away from your home and children is without doubt the biggest mindf##k I have ever known and if it comes to this you’ll have my sincerest sympathy’s.
However hard it gets they WILL always be worth it.
Hope thats not all hyper preachy but having had to fight to keep contact with my son I know how difficult can be.