My policy has always been to speak the truth, but in a simplified, accountable and age-appropriate way. When they ask questions, I make sure I only answer what they've asked. As the kids have gotten older, they've asked deeper questions as they can handle it. We've always had very open and upfront conversations, so they are quite comfortable asking anything.
I wasn't clear from your post if your ex left and no one's talked about it at all in this past month or if you did talk to the kids before he left and now you are wanting to tell the kids it's going to be more permanent.