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I'm very sorry to hear about your situation. You wouldn't have standing over the custody matter, but you could seek to have the restraining order "quashed" or dismissed.
This wouldn't legally be defamation because of the "judicial proceedings exception" in defamation law. That is, things that are said in court, as part of pleadings, etc..., are "privileged" from being defamatory statements. If you could prove that they made some sort of false statement of fact outside of court, to a third party that resulted in quantifiable economic damages, you could then have a case for defamation. But there's not a cause of action for defamation related to applications for restraining orders (due to the judicial proceedings exception). Because there's not a lawsuit that you can bring, there's no statute of limitations.
The only thing that you could do is seek to quash / dismiss the restraining order. Other than that (and I'm really sorry to say this) you wouldn't have recourse against the parents that sought the order in the first place. The right to petition the government for redress of grievances (i.e. sue someone) is pretty fundamental, and courts don't recognize a separate cause of action that you can sue someone for bringing a legal action against you.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (good or better). Thank you, ***** ***** luck to you!