If I was in your situation this is what I would do.
I would stop the witnesses from having any kind of study with or even access to my kids.. If that is not possible due to their mother having custody and control, I would then insist on being present at every study.. and then instead of letting them conduct any kind of Bible study.. I would request that we actually study the whole formation and true history of the witness organisation and its published teachings over the last 130 years; and that right there will be your opportunity to take them completely to pieces and save your children from being swallowed into the cult.