It will kill children and adults with as little as 2 berries eaten!
The following is taken straight from Wikipedia
ToxicityBelladonna is one of the most toxic plants found in the Western hemisphere. All parts of the plant contain tropane alkaloids. The berries pose the greatest danger to children because they look attractive and have a somewhat sweet taste. The consumption of two to five berries by children and ten to twenty berries by adults can be lethal. The root of the plant is generally the most toxic part, though this can vary from one specimen to another. Ingestion of a single leaf of the plant can be fatal to an adult.
The active agents in Belladonna, atropine, hyoscine (scopolamine), and hyoscyamine, have anticholinergic properties. The symptoms of belladonna poisoning include dilated pupils, sensitivity to light, blurred vision, tachycardia, loss of balance, staggering, headache, rash, flushing, dry mouth and throat, slurred speech, urinary retention, constipation, confusion, hallucinations, delirium, and convulsions. The plant's deadly symptoms are caused by atropine's disruption of the parasympathetic nervous system's ability to regulate non-volitional/subconscious activities such as sweating, breathing, and heart rate. The antidote for belladonna poisoning is physostigmine or pilocarpine, the same as for atropine. Atropa belladonna is also toxic to many domestic animals and livestock causing narcosis and paralysis; however, cattle and rabbits seem to eat the plant without suffering harmful effects.