The problem is with proof. Proving that he is speaking ill of you to the children will be difficult. I know you say you do not wish to hire an attorney, but your chances without an attorney are very low.
You should request that the judge appoint an attorney for the children to represent the children. You can also bring out to the judge the issue of parental alienation. You should document the chronology of the interactions between the father, the children and yourself. I know that you stated that you do not want to hire an attorney, howeve, in a battle for change of custody and parental alienation an experienced family law attorney would be helpful.
Based upon the father's actions which are to interfere with your rights and his inability to forster a relationship between you and the children there is a good chance with all other things being equal that you can get custody.
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This advice is not meant to create an attorney-client relationship and is a general anwer to the question posed.