Although nowadays 塞豆窿 is a cute and fun word ( not offensive at all today ), it has a few grim theories of its origin:
[1] closed-minded, unreasonable elderly were called 塞豆翁, and eventually kids who were naughty and unreasonable were called 塞豆隆.
[2] in the old days, superstitious people would sacrifice children's lives to fight the problem of flooding, by stuffing children into the dam's drainage (豆窿).
[3] in the old days, sometimes the drainage of a fishing pond was clogged, and fishermen realized the reason was a child's body got stucked(窿) at the gate(竇) of the fishing pond. In this theory, people believe 塞豆隆 can be used as a curse to wish for a child's death.