My situation is similar as I am a single father of a 12 year old daughter. I (we) stopped attending meetings about 9 months ago and just about 100% of my extended family are JW's. I travel now and then for business and leave my daughter with my grandmother or her aunt/uncle (on mothers side) sometimes when I do. They take her to meetings and she basically does all the JW stuff with them.
What's worked for me is that I know what they're pumping into her head and I know what JW's teach and believe. I then teach her the REST of the stuff JW's believe, like "What do you think of your friend *******? Do you think she's bad? What would you think if Jehovah planned to kill her? We'll, did you know that witnesses believe that God is going to kill everyone who's not a witness at Armageddon? Do you think that's nice, or fair? Is that how you want to believe and what you want to look forward to?
I realize that might be harsh talk for some kids, but my daughter can handle that type of discussion well. She seems to really understand the strange and cruel things that JW's teach even though my family never tells her about it. I do. I feel it's HER decision what she does in her life and I just want her to have all the facts. At this point she has no desire to be a JW and feels their teachings are very cruel and strange. But she's respectful to JW family and does what she's told when with them.
I felt that if I just insisted she not be talked to or anything about it from family, she (my daughter) would see that restriction and later in life might become interested again. As of right now she expresses regularly how happy she is we are not JW's and how she doesn't want to be. Hopefully, that will continue.
I think the critical part is to maintain a very open relationship with your child so they are always ready to open up and discuss whatever is on their mind. I never quickly react when she says stuff, even if it shocks me. Early on she told me she wanted to still go to meetings with family. I told her that's fine and that's her decision, but we talked more and discussed it and a couple of weeks later she had changed her mind. I want her to know that she can ALWAYS talk to me about ANYTHING. Without that, I think it would be very difficult to protect her from their (JW's) influence.
Just wanted to express how I handle such a situation in the case it's helpful.