A precautionary principle that applies to all plants:
All house plants should not be within the reach of young children. Some of them are very toxic. If your young child has eaten a part of a plant, consult a health specialist.

Poinsettia toxicity :

There is a persistent myth that the poinsettia is highly toxic and even fatal for young children or pets. For that reason, many people deny the pleasure of having a beautiful poinsettia for the holidays. For the majority of the people and animals, the toxicity of the poinsettia is very light. In fact, if your pet ventures to bite a poinsettia leaf, it will find the first taste of the plant very unpleasant. In many cases, the animal will only salivate. In the worst case, if the leaf has been swallowed it will vomit or have a diarrhea. Generally, it is not necessary to consult a veterinarian.

For young children, the unpleasant taste of the leaves will ensure that they will not try to eat a lot. Poinsettia toxicity is generally slight. For some children, when the plant is ingested, it may cause vomiting and diarrhea. The latex of the plant can irritate the skin or cause allergies in sensitive people.

In most cases, the experience of biting into a poinsettia leaf is limited to a very unpleasant taste in the mouth.