As an adult visitor to their class, students will likely find you intimidating. If you stand while you help students, you might unintentionally tower over them and reinforce that intimidation. Instead get at students’ eye-level or lower. Make sure that you give students suggestions and never touch their keyboard or mouse! Try to remember that coding can be really difficult for beginners and avoid saying things like “this is easy.”
Example: "Can you talk me through what your code does?"
See this and related tips as a Tip Sheet: http://csteachingtips.org/Tips-for-classroom-volunteers