"Laudanum" was a widely available over the Counter"proprietary preparation consisting of opium dissolved in alcohol in Victorian and Edwardian times. It was intended as a patent medicine for pain relief, but was also widely used recreationally. A few drops in a baby's food or drink would certainly ensure sound sleeping ! but at the risk of setting the child on the road to opium addiction. I believe both Laudanum and Morphine, along with cocaine, were still legally available over the counter in the U.K until sometime in the'20's !!
As to the Headington Morris Dance of that name I have no idea how that came about, (possibly as a mishearing of a more correct name), but when I was dancing with Hartley Morris back in the '60's and 70's our squire Bob Tatman always introduced it as a "Soporific Dance" !!!!