Looks like a tale or a misidentification. Rubbing (applying/firmly pressing) hemlock against the skin would lead to blisters, not to death, unless rubbing is applied directly on skin cuts. If the wikipedia history is true, the real poisoning was secondary and probably inner (Either for breathing in a place with lots of plants, or accidentally shallowing small chunks, or a crime disguised as accident)

My bet would be that they mistoken it for something edible/medicinal and tried to make an antimosquito recipe. Children always take a quick bit of anything that their parents show them as interesting.