S.W.
I agree that for an 8 year-old, this is a grieving process. Nothing wrong with an ice cream celebration, but I would expect that getting over this will take a little time. Allow her to feel bad about this, listen to her. Then occupy her with eight-year-old things.
My stepdaughter moved out when she was 25 and my 8 year-old daughter was sad for a time, but soon realized that she didn't miss the "mothering" that her big sister tended to do. Now they talk on the phone and sometimes Skype, and it's OK.