I'm not one to minimize the health hazards of tobacco smoke (first- or second-hand), but there's nothing here to suggest that the toddlers' unruliness wasn't caused by general parental irresponsibility, for which smoking during pregnancy is a pretty solid marker (and which also has some genetic roots in many cases). There's lot of solid research showing the harm that tobacco smoke can cause, but this is not such research.
Are you implying that all those of us in our 40s and 50s had irresponsible parents, because the predominant number of us were born to mothers who smoked?
The prevalence of smoking has declined drastically since the mid 60s, yet there seem to be far more behavioral problems among children of today than of that era.