Sorry to hear about your trouble seeing the children.
The fact pattern here is a bit unusual, at least with regard to fathering the child. At any rate, the question you present appears to be whether or not withholding visitation amounts to undue influence or coercion.
In reality you have a legal right to enforce your custodial rights by filing an action for custody/visitation. Ideally you would have done this prior to sign any custodial agreement. The fact pattern you describe does not indicate any criminal activity on her part.
The good news is that you may still be able to file a custody action in Court regardless of any written contract or agreement you signed. You will need to at least consult with a local family law attorney however to review what you have signed and its affect. An attorney is not going to be able to advise you based upon an unseen agreement and fact pattern.
You can file a custody action yourself if you cannot afford an attorney. I have resources on my web site and articles about filing your own claim. Best of luck to you.
The information contained in this response is general information and is not legal advice. For more info, see www.doyledoylelaw.com. Nothing contained in this response should be construed as legal advice or creating an attorney-client relationship between the attorney and the reader. An attorney-client relationship may only be created by express representation agreement between Glenn Doyle of Doyle & Doyle, PA and Client.