Small children can remember trauma, so perhaps every time Elly started screaming at Farley it traumatized Elizabeth, so she remembers it even though she was very young.
Mothers tend to forget all the bad stuff that youngsters do and just remember the good.
The reason they do not show Elizabeth’s face after Elly says that is because Elizabeth is doubled over in laughter. Elizabeth and long time readers know that Elly forgets nothing.
At least no animals were killed in the maiming of the wallpaper.