I can answer you from the perspective of someone who has served a guardian at litem for children and a family lawyer for many years, but disclaimer, I am not a psychologist or child specialist and it would be wise for you to speak with one about what is happening with you and your children. I've seen parental alienate in varying degrees, but in your case, it sounds like your children's mother is engaging in a campaign which may be destroying the children's relationship with their other parent. If she is not allowing you to see the children during the times that you are entitled to see them under a Court Order, then you need to speak with an attorney about filing an action for contempt. That you have an arrearge in child support, if you still have one, is not a basis for her to withhold visitation. If you have gotten current on your child support, then it would be wise to put together copies of your payments to demonstrate that fact, should she decide to bring an action for contempt against you. Most importantly, go speak with an experienced family law attorney, bring any custody and support Order, proof of payments, and the texts / tweets / emails or whatever proof you have of the Mother denying you access to your children - and denying them access to you. Best of luck.
This answer is offered for informational purposes only. It is not offered as nor does it constitute legal advice. This answer does not constitute an attorney-client relationship. Do not rely on this answer in prosecuting or defending against any criminal or civil legal action. Speak to an attorney in your area about how to protect yourself and your interests.
The definition of "parental alienation" depends on if you are speaking in a legal sense, or psychological. The main concern/harm is in the emotional and mental damage alienation bestows, but I do not know that definition. In a legal sense, what you have described fits.
If you are indeed behind in child support, you of course need to pay the arrears. If you do not think that you are, your ex is only allowed to "enforce" her position by use of a contempt action. Under the law, your failure to pay support (if you have in deed failed) is not a justification for withholding visitation.
More important than explaining the term is you doing what you need to do to build and maintain a relationship with your children. That starts with taking advantage of your rights under the law. If you have a custody order, you need to file an action for contempt. If you do not, you need to get one put in place. (For either type of action, the emails and text messages would be a good tool for supporting your position.) In either event, it sounds as if you need to retain an attorney right away.
~ Kem Eyo
The above answer is a general explanation of legal rights and procedures. It does not constitute legal advice. Nor does it establish an attorney-client relationship between the individual posting the question and the attorney providing the answer.