Panel 1: As she and Liz walk down the hall, Elly marvels that April's vocabulary has expanded in her absence and can't believe how much she's changed in the two weeks she was away.
Panel 2: Liz reminds us that she's a stereotypical dumb kid when she uses lazy child language to state that April can walk downstairs and take off her own socks.
Panel 3: We set up a punchline at children's expense when Liz marvels that April has boundless curiosity and always wants to try new things.
Panel 4: When they pass by the room of The North American Blithering Idiot, Elly's mood turns into one of stark horror when Liz asks how long it will be until April behaves normally.
Summary: Liz's lack of curiosity is fairly easy to explain as we had a child with undiagnosed myopia interacting with a world in which there is a Looming Shape Of Imbecile Malice that struck without warning and was never chased away. Mike's lack of curiosity is trickier to explain. Perhaps it has something to do with him trying things only to be slapped down by a Looming Shape Of Belligerent Self-Pity that saw everything he did as being bad and wrong.