Hi, I'd like to help you with your question.
I am not surprised you feel a great sense of injustice. This situation has put you in a very bad position.
I am not sure who told you that it is not your story to tell, but there is no rule that says you cannot tell what you know. That is not to say it is ok to tell everyone what the babysitter shared with you. That is most definitely her story and not to be shared, since she trusted you with it. However, there is no reason you cannot warn your children against what happened. Especially considering that they may have suffered through abuse with him and are too afraid to tell.
Has your ex admitted to this abuse? This is important since you will need to balance telling others about this behavior and your ex's ability to call it as your word against his. He could also make it a legal issue if it is not true
Abusers count on secrets and threats to hide their behavior. That is often how they get away with abusing more than one child or person. It is no wonder your ex feels he can present himself as a great guy. He never paid for his crime and he has somehow bought your silence.
I would highly recommend you discuss this again with your therapist. If your therapist is the one who recommended you keep your ex's behavior a secret, I would seek out a second opinion. You need guidelines on who you want to tell this information to and how to tell it. You want to be careful who you share this with since it is powerful and could cause ripples through your family.
Also, it sounds like to me that you had therapy with your ex included in the sessions. I find this highly irregular. If you are trying to work out your issues about your ex, why is he involved? He should not be. That can only hinder or stop your chances of recovery.
I can understand everyone not wanting to involve the babysitter and protecting her identity, but keeping this behavior quiet otherwise rewards your ex for his behavior. Going out and telling everyone you know is not an option (your ex could charge you with slander if you have no proof of the behavior), but telling those close to you, like your children is alright. Just try to balance the risk of alienating your children with keeping them (or your grandchildren if you have them) safe. Be aware they may not believe you, or become angry with you, especially without proof. You can judge for yourself if you feel it is worth the risk.
I hope this has helped you,