Tell her that you BOTH need to talk to the kids together about this, before she files for separation, it's the fair and honest thing to do with them, they're children, they didn't decide to be born and didn't get a choice in whether or not they were born to parents in a rocky relationship. Regardless of what happens between the two of you, you owe the children your honesty and you guys should maintain civil relations first and deal with your crap afterwards.
After you talk to the kids about the separation, then she is free to file for separation and move out and do whatever she needs to do.
