Amateur astronomers are scattered all over NZ and are usually remarkably helpful, if a little modest or eccentric sometimes. Or you may find the children are not used to going outside at night, and need some coaxing and assistance to make worthwhile observations. If you have a genuine interest and it shows, then you are halfway there. Add a telescope, as large as is practicable. They are not easy to use, so practice days before the event.
Quality is important.
Posters are great teaching tools - eg spaceflight, solar system, star fields, galaxies, nebulae. 3-D models are essential for modelling the Earth Moon and Sun. Add a set of slides of the night sky and some of the objects in it, shown in a DARK room. Pipehenge is a great tool. It provides a frame of reference with which to visualise the patterns of movement in the sky.