When that third child is being born the second child is already beginning to reproduce (if you reproduce like Yeast). Clearly, that second child is more important because it has already given you a grandchild when your third child is still in the process of being born. That second child has already resulted into two offspring, and counts for twice as much.
That only matters at a specific moment in time, but if you take a slightly longer view that 3rd child had an enormous impact.
> That second child has already resulted into two offspring, and counts for twice as much.
Only for one generation, but we care about many many generations. 10 generation down the line the slight difference in timing makes no difference.
But the increase in number made an enormous difference.
You really need to sit and run some numbers and you'll see you are mistaken.
Maybe your intuition will help you if think of it as a steady state - babies in (born), babies out (die), rather than thinking about one moment in time. In a steady state the timing make no difference (because you've already waited out the time) - only the number matters.
> Clearly, that second child is more important because it has already given you a grandchild when your third child is still in the process of being born. That second child has already resulted into two offspring, and counts for twice as much.
Well, to pull the idea from my own other comment... a child+grandchild pair is not twice as good as a child even if all you care about is that specific moment in time). It's 1.5 times as good, because a grandchild is only half as good as a child.
And (as ars says) once all concerned have died, the timing offset is of no significance whatever. However, for the impact on your total representation in the world of the present (no particular present), which does have significance... see my other comment, where you're still quite wrong. The Euler-Lotka equation values all members of the population equally; that has no relationship to reality when you're talking about (in your own exact words) "how many copies your genes make of themselves".
How does that change the conclusion? 3 children vs 2 is still immensely more children, not just a marginal amount as you thought.