Panel 1: As John puts Lizzie in her crib, Mike asks exactly how long Elly will be gone. His response: six 'sleeps'.
Panel 2: Instead of reminding him that 'sleep' is baby-talk, Mike says that that's a long time. John replies by telling him that it isn't and it's going to go by fast. This is because John doesn't want to remember that children perceive time differently than he does. It'll take until Mike is thirty-three for him to think that six days zip by like they were nothing.
Panel 3: Failing to realize that he just told his son he was wrong to perceive time like a five year old is supposed to, John instead reminds him that he's got school and chores and they're going to eat out and see movies.
Panel 4: As Lizzie screams "MaaaaaMAAAAAAAAAAA", Mike asks what they're going to do after that.
Panel 5: We next see the three of them in bed as John thought-bubbles that six days is going to be a looooooooong time.
Summary: The basic and annoying premise is that not only is John foolish to think that he can take Elly's place but that children are wrong to perceive time the way they were designed to. To a five year old, six days is going to feel like six weeks would to an adult. (Plus also, the children are, for some reason, terrified of him and he's too dense to realize that Elly has poisoned their minds against him.)
ETA: We have yet another seasonal banner to puzzle over.