Yes, oleanders are toxic: IF you eat them, they will make you sick and if your toddler or cat eats them, they will be more sick, but won't die unless they somehow manage to eat huge quantities.
No, they don't die in winter in warm climates like yours. They would die in MY winter, which is why I don't have any. Your Nerium is rated down to zone 7b and you are probably in zone 9 or 10 (the higher the number, the warmer the winter; you can find your cold hardiness zone on various websites that will tell you zone by zipcode). You can leave it outside. It won't bloom in the winter (I think), but it will be fine. They are evergreen, so they will keep their leaves and stay green all winter.
Yes, if it is going to be in a container, it needs to be more or less the largest you can get - these are large shrubs with a mature size of more than 6 feet tall and almost that wide, so they need plenty of soil support.
However, if I lived in California, I'm not sure I would be buying oleander now (though I think they are beautiful). Much of Southern California and Arizona has been hit by an epidemic of a disease called oleander leaf scorch, which is incurable. Oleanders are dying by the thousands and the disease is spreading. I don't know if it is in your area yet. It is spread by an insect called the glassy-winged sharpshooter, which I think is in the whitefly family. Before you buy, talk to a knowledgeable nursery person (NOT at a big box store) about if the disease is in your area or if it is near. Then keep a close eye out for the insects or any disease symptoms.