#1 objective is GUARD THEIR HEARTS AS MUCH AS POSSIBLE. As much as this particular change is about Mr. and Mrs. SP...it is going to be a DEFINING moment in their LIVES. So... that's how I would frame my moves.
Suggestions: *In kid-speak, as clearly as you can, explain what will physically happen (movers coming, stuff going out of the door, where it's going, WHERE THEIR THINGS WILL BE). Keep it simple, SP. Then let them ask what they want to know beyond what you present. Keep answers simple and only answer what they ask. If you don't know, say "I don't know." Keep eye contact with your babies. They'll be watching for signs in YOUR eyes.
*I know you wouldn't...but just in case the moments get the best of you, DO NOT SMEAR MRS. SP at all to her children. This matters to THEM. My parents are divorced - my father has NEVER said an unkind word about my mother. My mother never misses an opp. to slam him. Guess who I respect more.
*Take them away from the house while this whole operation is going on - start to finish. Do something fun, meaningful.
*Bring something home from the get-a-away so there is diversion/distraction when you walk in the door the first time. Something that needs to be eaten quickly, a puppy or kitten, a bird or turtle, something to plug in and do immediately ~~~ ease them into the space with a distraction.
Just suggestions. None of this will make it easier for you. But the goal is to help the little SPs - as you know.