The braying false note emanating from that particular strip comes from the fact that children his age don't know or want to know how adults spend their time. That being said, it still does work to a degree. That's because John and Elly are, as I've said before, rather poor salesmen for maturity. This, of course, is owing to the odd fact that they feel as if they dare not be seen to enjoy family life lest they not be treated as sincere and serious people. While they do laugh at their children when they cannot be seen to laugh, the only thing that Mike, Liz and April ever see them do is frown in either rage, disappointment or terror so it's not a big surprise that they wouldn't be looking forward to growing up and being weighed down by all the burdens Mommy and Daddy won't shut up about.
What makes this all the more pathetic is that neither adult is smart enough to realize this and go out of their way to not understand that the reason the children don't want to be responsible is that they don't want to be joyless automatons waiting for the sweet release of death.