I don't think doubts are enough, Oz. I grew up in the Borg and had doubts my whole life but didn't do anything to get out until my life was half over. Most of my life I figured the Dubs were between 80 and 90% correct and because of how they demonized anyone and everyone non-JW, I also believed that nobody else was even close to that level of "truth." The Borg was perceived as the best a human organization can do in this system.
In my experience, I was only ready to chuck the entire thing after I learned that not only were they wrong about some things, they are wrong about almost everything AND they oppress their members via mind control techniques. It was the combination of being personally offended and KNOWING that my life was NOT at risk of eternal damnation for leaving that allowed me to go.
For a teen, the lack of freedom is a drag but as long as they believe both that the Borg control is harmless and that it is for their own eternal good, they will stick with the group.
Are your kids into the Internet? Maybe some time when they are visiting and it's a rainy day (so you're inside) or maybe before bedtime, you can watch some youtube vids together. It doesn't have to be anti-Dub or 100% educational. Check out some music videos or funny clips along with the important ones that can be found with simple searches:
Also search for Stanley Milgram, Mind Control, and stuff on thinking clearly based on evidence rather than on tradition, authority, or revelation.