why one-year-olds are not generally left to look after their younger siblings

Robin is fond of her doll, and is very enthusiastic, if not very cautious, about looking after it. In a space of less than five minutes, Robin had dangled the doll upside down by one leg, gripped it by the neck, "burped" it with sufficient vigor to break its spine, dropped it, attempted to stand on it, accidentally kicked it when another toy caught her eye, put it to bed on the extreme edge of the couch and almost asphyxiated it with an absolutely filthy blanket that she'd pulled out of a nearby basket of dirty laundry. You know, so it wouldn't get cold.