Ask Mr. Smarty Plants

Mr. Smarty Plants would like to divide this question into two parts; first we'll talk about toxicity, and then we'll discuss plant choices.

There are numerous databases that contain the names of plants that are toxic to diffferent domestic animals, and I have included several below. When you use the database, it is better to use the scientific names for the plants; Platanus occidentalis for the Sycamore, and Populus sp. for the Poplar. Not finding the plant name on the list is not a gaurantee that the plant is non toxic, but it increases the probality that it isn't.

Poisonous Plants of North Carolina

Cornell University Plants Poisonous to Livestock

Toxic Plants of Texas 

University of Pennsylvania Poisonous Plants

ASPCA Toxic and Non-toxic Plant List - Dogs 

When choosing a tree to plant, think about these things; what do I want it do? (shade or ornamental), how much space do I have for it (think about mature size)?; how much "mess" can I tolerate?; what is the normal life span? By perusing this site from the Washington State University Extension, you will gain some insight to answering these and other questions dealing with planting trees.

 Hybrid Poplars have become very popular in the US because they are a fast growing tree with a pleasing columnar mature shape. However, their rapid growth is coupled with a short life span. Generally, they represent crosses between North American Cottonwoods and European Poplars. This link gives some pros and cons regarding planting hybrid Poplars.

The American Sycamore Platanus occidentalis (American sycamore) is a fast growing shade tree that is drought resistant, deer resistant, and can grow to  height of 70 to 100 feet; a very large tree! This WSU Extension site recommends against planting this tree

I'm going to refer you to a couple of previously answered questions; the first one has some good information about other fast growing shade trees for Central Texas, and the second has contains tips for when you are ready to purchase your new tree.