I have a couple of "hands-on" activities for your class. We can make an astrolabe, which is used for measuring angles above the horizon to stars and planets, (and with a tape measure and a little math can be used to determine the height of buildings and trees and such). The other is simply using a stick/pencil/crayon and their hand to tell time--the Hand Sundial.
Here are some materials which you may download and have prepared by your children for my visit:
For the "Do-it-yourself-astrolabe" each child will need:
1) a six inch diameter paper half-circle
2) drinking straw (best if non-bendable)
4) 6" to 8" length of string, and some clear tape
5) some sort of counter-weight--a marker with a cap will do in a pinch
Using a protractor, have the children mark off ten degree intervals, or more if they wish, with Zero at the bottom of the curve, and 90 at either end, thusly:
For the Hand Sundial, all that's needed is a straight stick or pen, knowledge of north, and a sunny day.
We can also try to sketch the moon and sun, if the numbers of children are managable. All that's needed there would be a white page with two circles spaced apart on it.