They are quite different in their mechanism of action. Caffeine works via Adenosine, Nicotine works via Nicotinic Acetylcholine receptors.
Nicotine is one of the most toxic of all poisons and has a rapid onset of action. Apart from local actions, the target organs are the peripheral and central nervous systems. In severe poisoning, there are tremors, prostration, cyanosis, dypnoea, convulsion, progression to collapse and coma. Even death may occur from paralysis of respiratory muscles and/or central respiratory failure with a LD50 in adults of around 30-60 mg of nicotine. In children the LD50 is around 10 mg.
- Eating or drinking inappropriate things,
- Eating or drinking things which are colorful or have a pleasant aroma, and
- Being more sensitive to certain chemicals (e.g. nicotine) than adults.
So if you told me, "Let's put nicotine in a mango-scented liquid and put it in reach of children," I think we should be alarmed. Likewise, when we took iron supplements and put them in the shape of colorful pills or animals with a sweet taste like candy, and then children started eating them and getting iron poisoning, we got concerned.