While poinsetta plants aren't poisonous to cats in that they will die from it, they can get very sick if they ingest the poinsetta sap found in the leaf and stem. They could start to vomit and become irritated in the mouth. So, keep an eye on them to see if they are reacting in any way. Just keep the plants in an area where they are less tempted to eat it.
From what I've investigated, poinsetta can make cats ill but will not kill them. Cats tend to not eat vegetable matter anyway, except when they feel ill and then they eat grass to make themselves vomit.