This is because we're dealing with a situation in which we're dealing with Lizzie, having listened to Daddy's silly, cartoon-like explanation, quotes what Teacher says in a very serious tone of voice because she's sort of too tiny to realize that Daddy is simply messing with her and thus seeks to correct him because she doesn't know better. Since John and Elly started out AND ended the strip not understanding how children think, we're dealing with a situation that has him being filled with despair and horror when the appropriate response is either a bemused smirk or a rueful chuckle about kids say the darnedest things. You and I see a five year old not quite understanding that she's not as well-informed as she believes herself to be; Panicky Nitwit John's flight-or-fight instincts have been advocated because he wants to believe in monsters.
Oh, he certainly does dress up his fear by making a different sort of panicky noise about how extra-bad people are going to take advantage of the children when they get out into the real world and think that they know more than they actually do but his and Elly's soiling their Depends about the dangerous information that on the Internet that the Government should censor because it's the State's job to raise the kids tells us, what he fears and distrusts are the children he created because, as I said, he lives in fear of a bogeyman patched together not in Doctor Frankenstein's lab but in a television news outlet's conference room: the teenaged superpredator. Why else do he and Elly call the teens "THEM" as if they're fleeing from a cicada the size of the USS Missouri?