Oh, explaining to Simon has been way easier. He's pretty much on top of things. Irene, not so much.
Meanwhile, one of Simon's former classmates, who I think of as his "best frenemy," sent the message through her mom--one of my Facebook friends since the kids were in preschool together--that she missed him. I told him this yesterday, and Irene responded with, "She misses me, too!" Ellie's mom says that Ellie came back with, "I can't remember her name, but sure, I miss her. I miss everyone."