Although a number of very common plants can be considered to be poisonous to cats, spinach is not one of them. Your cats should be fine eating spinach so you will not have to worry about them dying or even getting an upset stomach as a result, though not all cats will react to eating spinach the same way. Eating enough of any food at all is enough to upset a person or an animal’s stomach.
Spinach actually has a lot of very valuable vitamins, nutrients, and minerals which you and your pet will be able to benefit from. Once in a while you might want to throw a little bit of raw spinach into your cat’s food bowl just for flavor in order to add some variety to what they normally eat.