Also, having kids helps you live longer? Did they take into account the physical, mental, and financial strain that parenthood can bring? I'm willing to bet 99.999% of parents are scratching their heads over this one too.
Not sure which part of the world you live in, but the study looked at Swedes, who typically don't have to worry about tuition fees, health insurance, maternity/paternity leave and other common U.S. parenting challenges.
That said, what other posters have already suggested about children providing positive experiences to your old age lifestyle is the most likely mechanic behind these observations. Not sure I would find enough reason to stick around if I found myself unable to enjoy life due to geriatric problems and no one left to mourn my passing.