I suggest that you not take a hardline stance against your children studying with Witnesses. Rather, be open and supportive. Also, get them involved in wholesome activities that Jehovah's Witnesses oppose. Make sure they play sports, go to their friends birthday parties, go trick-or-treating at Halloween, and have a wonderful Christmas.
As your children get older they will not want to go to meetings, go out in a door-to-door ministry, of give up fun activities for a control group. By allowing your children to study or go to meetings because THEY want to it gives you a strong foundation to support your children and oppose your in-laws when your kids no longer want to do it.