they preaching to my children, behind my back? Is this the way JWs operate?
Yes, it is the way they operate. They will do everything they can to turn your children and wife against you, in a spiritual sense.
In fact, your wife and children are even now being primed to look out for your objections. Scriptures about 'setting parents against children' etc will be cited to 'prove' that such opposition means that they have the 'truth'
Tread carefully. Outright opposition may be too late, it may make matters worse, you'll have to play it by ear. I suggest asking them lots of questions about issues that JWs don't want newbies to know about. For example: if children\wife become JWs will they be able to have friends in the world?, have birthdays?, celebrate christmas?, have to preach door-to-door?, have blood transfusions after a serious accident?, question direction from the GB?
Depending on how advanced into it they are you might bring up child protection issues, the UN, and failed prophecy. That requires a lot of research on your part.
Once the indoctrination fully sets in they will not answer or even listen to you about such things.
I wish you the best of luck.