My guess is wrong: Elly is messing with John's head to watch him squirm and yelp.
Panel 1: We start off with John, having been told of Annie's clever plan to alleviate Elly's boredom, tell Elly, who's carrying Lizzie in her arms, "Another kid?! We're not having another baby, Elly."
Panel 2: She asks if he's positive; he replies that he's absolutely positive.
Panel 3: Her relief that he actually doesn't want to be intentionally invaded by Mars inspires her to smile and tell him to take permanent precautions.
Panel 4: This causes him to yelp that he's not THAT positive.
Summary: John is evil and cruel because he doesn't want to be oopsed at some later point; he's also evil and cruel because at that later point, he doesn't mind being oopsed.