This kinda bugs me, not only because inter-generational child rearing has been the norm for most of history rather than something unique that deadbeat millenials are relying on, but also because it propagates the belief that anything you don't pay for with money is considered "free" (the bad "free", as in "deadbeat" or "moocher").
There's the whole debate that GDP is the wrong measure precisely because it doesn't measure these kinds of "unpaid services", but I prefer to look at it another way: a "livable" city is more than what restaurants and bars are around, but also who your neighbors and social connections are (I've heard this described as alternative forms of capital beyond financial capital like social and cultural capital).
It's not that millenials are killing the babysitter industry, it's more millenials are reaching the age where they understand the whole "it takes a village" folk wisdom.