The most beautiful plants could possibly be poisonous if consumed by humans.  You want to be careful when planning your garden, especially if you have small children.  Children have a tendency to put things in their mouth, and there are some plants that can cause serious health issues if consumed.

Hyacinth, Narcissus, and Daffodil bulbs can be poisonous if eaten (not only by humans, but also pets).  Some berry producing plants can also be harmful like holly and mistletoe plants.  Sometimes it is the leaves that are harmful with plants like Elephant Ear.  You have to also be careful of some fruit seeds or pits.

If you are suspicious that someone has consumed a plant that could be harmful, or even if you are not sure if it is harmful, you should contact a poison control center.  Knowing exactly which plants are in your garden is important in case an incident occurs.  Below are a couple of links that identify some known harmful plants (one by Cornell University and one by North Carolina State University) and a link to the American Association of Poison Control Centers.

http://www.ansci.cornell.edu/plants/index.html

http://www.ces.ncsu.edu/depts/hort/consumer/poison/poison.htm

http://www.aapcc.org/