That age group is involved in production (e.g., in media.)
But not lot, so, yes, the narrow first-order GDP effect would be very small.
> With a second approximation, you might say that GDP would increase since parental childcare is not part of the GDP and those parents could instead be adding to the GDP.
I suspect second order demand side effects would probably be more strongly negative that second order supply side effects would be positive. You can't predict GDP effects from labor supply side effects alone.
Eventually though the benefit disappears as the demographic pyramid becomes a pipe and the share of working population shrinks again. The US demographic dividend was the era of the baby boomers who were a large cohort but had relatively few children.
Similarly, the current flatlining (ignoring the decrease due to excess deaths) of the population won't effect actual production until a couple of decades on, unless you count possibly increased production described above.