Hi Tricia and welcome to GoY. I'm not sure exactly what you are asking us as there does not appear to be a question. Yes oleanders are poisonous as are many of our common harden plants. I do not see that it was the TV companies responsibility to tell you this though. Children need to learn not to pick and eat anything that is in the garden without permission and animals are not foolish enough to eat poisonous plants.

Ask youself this simple question - Where are the bodies? As Moon_grower says they are very poisonous but there is a big difference between being poisonous and being harmful.

The number of incidents is very small not least because there's nothing about the plant that makes you want to eat it.

It is on the Horticultural Trades Association list of potentially harmful plants in category 'B' which is the highest category for all practical purposes so a responsible seller should identify it as toxic if eaten. That allows buyers to make a choice.

Since you were denied that choice, I think you should be able to return them if you wish.

As an aside, I'm not sure I'd try growing them as a hedge in the UK. They might thrive in the Mediterranean and other warm climates but they'd need to be kept sheltered here.

Thepoisongardener's last point is one I was going to make - your biggest problem with these is that they're not entirely hardy here. If you live somewhere warmer such as the west country or in the middle of a large city, and they're planted in a very sheltered and sunny spot, they may do okay. Although they often survive the winter, the flowering is poor, and you may need to cut back hard, so as for making a 'hedge', not a good choice. There are plenty of other plants which are hardy which will make a good small hedge.
As for its being toxic, it does have a potentially irritant sap on human skin, and most plants like this should carry a 'toxic if eaten' warning, but that said, you do have to bear in mind that over 50% of plants grown in most gardens here are toxic if eaten. Laburnum, for instance, particularly the seeds, will kill people if consumed. Rhododendrons are toxic, daffodil bulbs are toxic, Ruta graveolens (rue) may cause a serious and painful rash on skin contact, euphorbia sap is highly irritant to skin, and of course deadly nightshade, which can pop up unbidden in a garden - the list is extensive. But of course, we don't actually eat any of these things as a matter of course, and these plants are useful teaching aids for children when informing them of what's safe in the garden and what isn't.
This situation also neatly illustrates the point that its best to research the plants you're thinking about buying before making a choice, particularly if they are for special purposes, such as a low hedge.
All that said, I do agree that their toxicity should have been mentioned at point of sale, but I'm not sure they're legally obliged to do so.

I think that you might have a battle on your hands and as mentioned garden centres who are members of the Horticultural Trades Association would be in trouble from their governing body if, when inspected, they failed to state on their plant labels that they were toxic. I would say that the company from whom you bought the plants did not advertise the fact that they were toxic because they knew that most non-plants people wouln't buy them.