Panel 1: We start things off with the kids getting way too happy about the new car because they forget who Elly is.
Panel 2: They start to remind themselves of this when she boasts about having a newer version of the same make and model she used to drive; Mike points out that even the color is the same as the old station wagon.
Panel 3: Elly is just fine with that because she likes driving a practical, utilitarian vehicle that's for the family.
Panel 4: Since it's an extension of the house, the kids should be less surprised than they are when she screams WIPE YOUR FEET BEFORE YOU GET IN!!
Summary: Their shock is because they have the same memory problem she has. Every day, they wake up and blank out on the fact that Elly wants them to respect her property despite never having actually modeled that behavior.