You're in a bind there with the children. I agree w/ IC that the kids would do better being forewarned, esp. if W reacts in total shock & poorly. Usually, the best advice is that the parents sit down together to talk to the kids and reassure them. It still is hard on them, but they see that the parents are still a team supporting them.
That said, the reality is a lot messier, and often what might be best has to be sacrificed for what is expedient.
My instinct is that you should tell both your W & your kids this is coming, but not too much before she is served. Once you tell the kids, there is a chance that it will get to W (one calls distraught for instance). I'd tell her first, then the kids. Give her the chance to sit down with you to discuss telling them together if she can do it in a non-blaming way that puts the kids' needs first. That this day was likely coming should not be a surprise for her. You give her the opportunity to be part of the caring for the kids if she is able & willing to.
Me: 50 W:43 S6, S3 M: 12 yrs. T: 17 M is bad & Not happy Bomb Mar '14 S 5 Feb '15 D Bomb 13 Apr '15 (but "no hurry") DB Coach May '15 Wants proceed on D Aug '15 Starting 1-on-1 negotiations Sept '15