Interesting stuff.

... it was observed that women who had either no children or had more than four children had higher mortality
...
However these studies pertain to having a bunch of kids, having kids in general is still thought to increase lifespan (especially in that 1-3 child range).

Another possibility could be that healthier people with a stable social background and high education (who anyway have a longer lifespan) tend to have 1 to 3 children. The number of children may correlate with the length of the lifespan, but need not necessarily be the reason for it.
Edit (addition):
also some women with 0 children may have none because of some sicknesses (which reduce lifespan as well).

Interesting explanations why boys could reduce the lifespan in comparison with girls: a) they may cause more stress or b) because of different hormone concentrations depending on the sex of the baby.
Another hypothesis would be that mothers are more affected than fathers, because in average they still spend more time with their children than fathers, so that in case it is true that boys are causing more stress than girls mothers suffer more than fathers (in average).