Read the book "Raising Freethinkers - A guide to parenting beyond belief."
It's not easy but you got to teach them to critically think for themselves. You can't do that by aggressively showing them the bible is crap.
They need to be able to question things, even your own views. When they ask questions, you can help them to find answers with reason outside of jw thinking. But don't be dogmatic too. Make sure that they know that you support them even if they change their beliefs a thousand times. Give them a lot of love. It is a good sign, that your youngest one asks you questions.
When talking about darwin, you could show them the last sentence of his work "On the origin of species", which shows that his work is not against a belief in god.
I also have to counteract on the indoctrination my little boy gets from the cult and his mum (we are separated and soon divorced).
The above mentioned book helps me a lot ESPECIALLY CHAPTER 3. I am sure it will help you too.