It would depend on the twins.
Identical twins would indeed have identical DNA since they arise from one egg that splits in two and forms two humans.
Fraternal twins (two eggs, two sperm) would have DNA that would be somewhat similar but not identical.
Anything, technically, can be a poison. Digitalis in the wrong dose can be fatal to a child. Many plants are poisonous depending on which part of the plant (roots, berries, leaves) is ingested. I'm not up on plant poisons so I won't make any specific suggestions. A good pharmacognosy text book should answer all your questions in that regard, along with providing signs and symptoms .
There are a lot of non-organic things that can poison children, too. Almost any type of cleaning product - window, sink, shower, carpet, automobile - will have ingredients that are toxic if ingested. Researching the active ingredients on the internet should give you the symptoms of poisoning.
Hope this helps.
Dave