When you are talking about a real poison, certain poisons can be ingested in very small quantities and have very minor effects. Cumulative and long-term effects are another thing... take lead poisoning, for instance, or asbestos in a building.
The fact remains, however, that it is illegal and immoral to knowingly poison someone, or to know that someone is being poisoned by another party and to not do anything to stop or prevent further poisoning. Even when a person voluntarily "drinks the kool-aid", and is of an age to understand the consequences of it, it would be highly unethical for someone who is duty-bound (like a parent) to protect that individual to ignore such a serious act of self-destruction.
So, what do you do? If you go into "rescue mode", you run the risk of frightening the children into doing exactly what your ex spouse wants. He's probably already primed them to watch for Apostate™ talk and behaviour, and to report back to him. So you have to prove to the kids that you aren't the raving lunatic Apostate™ that he's told them to expect.
Help the kids to keep their options open. Try to get them involved with extra-curricular activities at school, like band, sports, the school newspaper, "reading buddies" with the younger children; things that aren't going to raise a lot of red flags, things that will help them socialize with non-JW kids and form some non-JW friendships. Let them participate in field trips. While my parents didn't want us to sing the National Anthem at assemblies, if we were practicing the melody in music class, my parents expected us to participate, and we had to know the lyrics too - same thing with holiday music - it was part of our education, not part of the celebration/observance. [The other thing was they figured, so soon after 1975, that we were sure to be arrested any day now, so if we could play that music and know the words to those songs, we might not get thrown in jail! LOL]
Children in their teens are still in the midst of brain development, too - that's why more abstract concepts are introduced in the higher grades, including the skill of critical thinking. If your kids have to do a project on current political situations, have them examine the issues under debate, and get them to do some fact-checking on the news stories before they take sides in the debate. Get them to think about things that appeal to young people - like animal rights, women's rights, civil rights, and so on. Talk to them about social injustice in the world around them - a good example is how women are devalued in 3rd world countries. At some point, a light bulb will go on in their own heads, about how the WTS devalues women too. Use examples from other cults (like the FLDS) that treats women like chattel, and ask them what makes that belief system a cult. They'll make the connection without you ever attacking the WTS.
This is something that can take many, many patient discussions. It might be a good idea to develop a kind of outline of the sorts of topics that your kids might be interested in, with some pertinent facts. You'll have to do homework when they aren't with you, to be prepared for a rational and reasonable discussion that allows them the opportunity to express themselves while you lead the topic of conversation.
It will make for a much better outcome for the children if you help them to gradually come to their own conclusion, than if you try to ram it down their throats in "emergency mode".