I can't advise you as someone who has seen his spouse move out, but I can advise you as someone who moved himself many times as a child (military brat ) and as someone who has moved his own children. I know this move is going to be REALLY difficult for you, but for a minute, I am putting myself in your children's shoes.
My memories and coping techniques as a little Thinker who kept moving was not to focus on the loss of the old, but on the gain of the new.
Remember that, difficult as it is for you, they are also moving. As I understand it, they are going to be spending part of their time with Mrs. SP in her new house, so this is also their new house as well.
With this in mind, my recommendation would be not to focus their attention at all on her moving out, but to allow them to be focused on themselves moving in to their new house.
Unfortunately, you can't be the one to focus them on this. Your W has to do it.
Is it possible for you to step back for a day, acknowledge that it's moving day but take them out and entertain them while the movers take everything out of your house, and then hand them over to your W so that she can "Celebrate" their new home with them. For their sake, she really needs to do that.
Your focus should be on distracting them from the move out.
Also, one of the most distressing parts of a move out for a child is looking at the empty hole where all of the familiar stuff used to be. Have a plan for how you are going to fill those holes and make your house still a complete home (for your kids) once her stuff is gone.
Just my opinion.
I am sorry that you have to go through this. I really am.
Me 42, W 39, S8, S6, S2 M 11y, A & ILYBNILWY 11/08 Walking away from a bad situation.