Unless there is a visitation/custody arrangement, there's not much you can do. Could you file for an emergency hearing to get temporary custody/visitation set up since she is denying you??
You sound like a great dad! I have been trying to get my ex to spend time with our son for years. I pay for son's cell phone so he can talk to his dad when he wants and have never refused his dad access to him, but his dad just doesn't seem to be interested. My son hasn't spent the night at his dad's house since January 2007, and his dad only lives 15 minutes from us. Sad, but true.
Keep fighting for your kids. They need both parents!