You have an interesting occupation. Try to relate to the students exactly what you do in a language appropriate to their grade level. Tell them why you believe what you do is important and why you would like to share your experience with them. Students believe you to be successful. Tell the students you believe yourself to be successful.
Students spend most of their day with frustrated adults.
The students would like to know there are adults who actually enjoy their work and think it worthwhile. There is nothing compelling you to be in that classroom other than your genuine enthusasium to share your knowledge with the students. Most students appreciate this fact. Use it to your advantage. The children know that you are giving of your time.