imo, if mom and/or dad at home are doing their job then protecting their kids against possible indoctrination of jw beleifs from other family members or strangers is no different than protecting them against any other of lifes dangers.
we already know how the jw mind works, we know their tactics, we know everything about them inside and out. I personally feel I have the advantage and there is nothing they could do to 'convert' my child because he living with me is protection enough for him. the one time they tried to convince him of some basic jw belief it turned out to prove me right because he challenged them with common sense questions which they were unable to respond to.
i think protecting my child from a cult whether jw or other is just another challenge in parenting. the only difference is i have the advantage since i know all about the jws that is a challenge i am more than confident i can face. i'm more concerned about my kid falling prey to something else in life something which i dont know about and may not be able to protect him against.