The belief that poinsettias are poisonous is false.
No one is sure how this myth began, although it is often attributed to the 1919 death of a young girl whose parents thought she had eaten poinsettia leaves. The truth is, a child would have to eat about 500 poinsettia leaves to get sick.
That is not to say poinsettia is harmless. If a child eats enough poinsettia leaves (say five), he may become nauseated or throw up. But he is definitely not going to die. And probably he is not going to eat more than one or two bites in the first place because the leaves are reported to have an unpleasant taste.