While eating a poinsettia may cause a child's mouth or rectum to sting and burn a bit, poinsettias aren't poisonous.  The following holiday plants are actually poisonous and therefore much more dangerous:
• Amaryllis
• Holly -- especially the berries
• Jerusalem Cherry -- leaves and unripe fruit
• Mistletoe -- especially the berries

If a child ingests any of these, you need to seek medical attention immediately.