Answer: It is difficult to come up with such a list, since even many "edible" plants are toxic if the wrong parts are consumed--for example, rhubarb leaves. I can list for you some common ornamental plants that are poisonous, that you should definitely avoid:
Autumn crocus, black locust, bleeding heart, box, buckeye, castor bean plant, cherry laurel, daffodil, daphne, English ivy, foxglove, golden chain tree, hydrangea, lantana, larkspur/delphinium, lily-of-the-valley, mountain laurel, oleander, privet, rhododendron/azalea, sweet pea, wisteria, yew.
I'm afraid that any garden would contain possibly toxic plants, so strict supervision would be necessary if accidental ingestion is possible. I would certainly avoid plants with sharp parts/thorns (yucca, roses), to reduce the chance of cuts and scrapes.
I hope this is helpful.
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