Unfortunately since you are not a party to the case, you have no right or avenue by which to remove the judge from your ex-husband's case. However, your daughter and you should have had a victim's witness advocate from the prosecutors' office contact while the case was pending. Call that person and express your concerns, or call the prosecutor who handled the case. While they cannot control the terms of the probation, if there is some hanky panky going on perhaps the prosecutor's involvement will stop it. OR, it may be that the judge is not really involved with the changes and it is recommendations of the probation officer who is biased toward your ex. The prosecutor could bring the issues to the judge's attention to put a stop to it.
You can file a "Motion to Recuse Judge." However, these are not normally granted. You should consult an attorney, give him all of the facts and why you believe this bias exists, and see what advice you receive.
You ask who to write to - I would instead set up an initial consultation with an attorney so you can sit down with a back and forth dialogue. Many times a simple letter or email or posting will leave out answers because questions have not been asked. Find an attorney through AVVO!
A motion to recuse judge, or the less inflammatory motion to reassign the case can only be filed by a party or quasi-party such as an attorney ad litem. If you are not a party to the case, you can't do anything about the judge who is assigned to the case.
If you are a party, the motion to recuse, which has to be sworn and contain specific allegations, will probably be granted unless the administrative judge thinks you're just wasting the court's time or trying to delay the case. Where I practice, most judges do not want to preside over cases that may just end up in a disciplinary complaint at the end of the matter.