Depends how you define 'poisonous'. Animals and humans who consume the plant won't die, but they may drool, experience nausea and/or vomit. Poinsettia has a bad reputation, but there are far more toxic plants that people might use at Christmas, see here https://youshouldgrow.com/are-poinsettias-poisonous-to-cats-dogs/
In regard to children, the taste of the leaves is not pleasant, so they are unlikely to consume a large amount - current estimates suggest that a 50lb child would have to eat 500 leaves to reach a possibly toxic amount. This scenario is obviously highly unlikely, especially with so much chocolate and sweet foods around at Christmas, no child will choose to eat tons of unpleasant tasting poinsettia leaves in preference. There's probably more risk of harm from a child crashing into the plant and getting the sap on their skin than from eating it, because the sap is irritant to some people.
The myth about their extreme toxicity arises from the case of a 2 year old child who consumed one poinsettia leaf, and the child's death was attributed to that, and so this story created an urban myth. The cause of the child's death was apparently unrelated to consumption of the leaf, more info on that and toxicity to humans here https://www.medicinenet.com/are_poinsettia_plants_poisonous_fact_or_fiction/views.htm
You might find the following of interest http://www.thepoisongarden.co.uk/atoz/euphorbia_pulcherrima.htm