Give the first graders a definition of what a zoologist is - a person who studies animals. Explain how these people study animals - by going to a college with an animal science program - and explain how a zoologist has to have a degree in animal study, so he most likely specializes in a type of animal.
Explain where zoologists work - usually at a zoo or other animal conservatory, or they sometimes work in a lab studying different types of animals. A zoologist might also work in the field, with a particular type of animal.
Watch a video of a zoologist in action, or better yet, take the first graders to a zoo or another animal conservatory where you can see zoologists in action. This will allow students the chance to see how a zoologist spends his day and get a better understanding of what a zoologist's life is like.