well, you seem like a real jackass. I feel sorry for those kids in the long run. Who care about a couple of fricking dollars here and there, those kids need a dad. If you are not sure if they are your then go get a test done to find out them own your butt up and take care of YOUR responsibilities. She didn't have those kids herself, you helped out, you need to be there for the kids, forget your X, your kids need you.
I am going through something similar with my X not wanting to see his kids, that is bullshit. My kids cry all the time wanting to see him but he don't care. He says he is going to show up and never does, no phone call or nothing. I have to deal with my kids heartbreak from him all the time. I SEE how it affects the kids and you do not need to do that to those kids, they deserve better then that.
I hope you grow up and do the right thing by those kids. I don't mean to be rude but I am thinking about the kids here, not the money coming out of your pocket.