All parts of the Christmas rose (Helleborus niger) are poisonous, but the plants are not appealing to cats or dogs. I can't promise that your pets won't be tempted to nibble, but it's highly unlikely. Other plants that contain the same kind of poison include marsh marigold (Caltha palustris), buttercup (Ranunculus), and baneberry (Actaea). As far as location goes, Christmas rose needs a shady location with a minimum of root competition. I suggest not planting directly under a maple or other shallow-rooted tree. Farther from the tree's trunk, but still under the canopy, would be a better spot. Improve the soil with organic matter, because the plant needs well-drained soil; it grows poorly in straight clay. You can amend small planting pockets without damaging the tree's roots.