Chocolate contains a chemical that is deadly to humans of any age in high doses. It's called theobromine, from the Greek 'theobroma' meaning "food for the gods." Which is fitting since chocolate tastes heavenly, and if you eat too much, you'll wind up meeting your maker.
Overdosing on theobromine can cause vomiting, diarrhea, seizures, internal bleeding, cardiac arrhythmias and even fatal heart attacks. That dangerous dose is around 1,000 milligrams for every kilogram that a person weighs, so consuming roughly 77 grams of theobromine could be fatal to someone weighing 170 pounds.
But before you put the kibosh on trick-or-treating this year, keep in mind that 77 grams of theobromine might not seem like much, but it's actually a lot. On average, one milk chocolate M&M contains a single milligram of theobromine. So a 170 lb. adult would have to eat 77,000 at once to hit that lethal threshold, and a child weighing 50 lbs. would need to scarf over 15,000 before you'd need to worry about how much their theobromine consumption. So the little chocolates handed out on Halloween aren't fun-sized death sentences by any stretch.
Unless there's a chocolate river flowing through your neighborhood like in Willy Wonka's factory, you're probably fine. Just like you'll be fine if there's a well-regulated dispensary or a budtender living on your block because the most dangerous part of a pot brownie isn't the THC. It's the chocolate.