Go to the courthouse and file for custody and visitation. You don't need a lawyer, just ask the clerk for the forms to complete. It will come with instructions. You don't need a lawyer (but it's easier if you do). You'll get a court date.
Are you paying CS directly to her, or through the state service? If you're paying directly through her make sure you have receipts for all of it, and that the receipts specifically say "child support
Also, start keeping track of when you have the kids and for how long. Back up the log with any physical proof you have. Try to go back as far as you can and document when you have had them. This will help your case in court.