Hi serenitynow
I don't think you could take legal action to stop your ex or his family from talking to your children about religion. In any case jws would think of that as "persecution" and they would rally round the family and kids even more.
I'd say don't stress too much - I can hardly imagine that the meetings and field service could be too attractive to your children - even those brought up as witnesses don't like those aspects too much.
Your ex must still be very brainwashed from his upbringing. However, your children haven't had that brainwashing so don't underestimate their ability to reason and see through things.
penny2