Consider setting up stations and have students rotate through a series of demonstrations or activities. Keep scientific terminology to a minimum. Don't assume any prior knowledge. Make your explanations simple and use words appropriate to the level of the student. Young students love learning new words, so don't leave out scientific terms altogether.
Safety comes first.
The teacher can help determine if there are safety issues you need to consider. Remember to bring gloves and paper towels. Determine what you need to bring. Practice makes perfect. Get feedback on your planned activity from colleagues, the teacher, and, if possible, children of an age similar to the class you will visit.