Am I to understand that, as far as you know, the kids do not yet know your W is planning on leaving, let alone taking them with her?
If that is the case, I think that your best bet is to let your W know that you feel it is important for all of you to sit down and discuss this with the kids.
You could let her know that you DO want to discuss this with the kids and are prepared to do so, without her presence, although that will probably lead to an argument.
At the end of the day, understand that, the kids will have a lot of questions and concerns regarding the move. The sooner they now about it, the sooner your W will have to deal with the fallout.
Just brace yourself to be the best dad you can be and be ready to support and comfort them in this stress. They will not understand and will look to both of you to support them.
Also, I would probably recommend that you speak to a L regarding this, ASAP. She may not have a legal right to move the kids without an appropriate custody agreement in place.
