What are my options to try and get this interfering with parent/child relationship to cease?

My ex wife and her parents have been telling my 5 year old how they do not like me or my family. She has been hearing bad words from them. I asked her where she saw that word and she replied it was on mom's phone whenever I called. She knew what the word was, without me telling her, as I asked her what she thought it spelled. My child has also told me how her grandparents say they don't like me, etc. She has punished my child for bad behavior by not allowing her to Skype with me. It states in our divorce that neither of us shall defame the other parent nor interfere with their relationship. The divorce was filed in Pennsylvania. Thank you.