Downplayed: Alice looks into her childrens' old bedrooms for a few wistful seconds before turning them into a computer room, a home gym, and a den, respectively.
A large part of Alice's identity is being The Mother. Now that the kids are not kids anymore and don't need her, she feels like she's lost that sense of purpose.
Alice misses the kids, and the happy memories (and those memories that maybe weren't so happy at the time, such as when Billy discovered his artistic side...and the newly-painted wall, but are now something to look back on and laugh at)
Lampshaded: "It's almost too quiet without the kids here."
Invoked: The youngest of Alice's children has finally gone off to college...and made new friends and found an apartment to live in.
An apartment or condo complex markets itself to older adults whose children have grown up, now that they have less (or no) need of all that extra space.
Billy is away at college and dating a Hard-Drinking Party Girl...and spent all his money on her. He tries to make Alice and Bob miss him so they'll worry and send him more money (and not fret about his appalling GPA).