Solving the ancient mysteries of astronavigation allowed the human race to achieve world-conquering exploration, all of which began by the simple task of telling the time of day by the position of the sun. Making a sundial with your children is a perfect opportunity to teach them about time, the Earth's rotation and the solar system, so quickly go to http://en.wikipedia.org/wiki/solar_system and learn all about it first.
If you live in the northern hemisphere you can find due south by pointing the hour hand of your watch at the sun, then finding the mid-point between that and the 12 on your watch. This is due south. Alternatively, look at a map of your street, or even better the deeds of your house, on which north is usually marked with a large N. South is the opposite direction. QED.
You will need
· A piece of wood about 300-400mm square for the base
· A jigsaw or padsaw (optional)
· A piece of 6mm plywood, cut into a triangle (150mm x 150mm x 200mm)
· Two 150mm strips of quarter-section doweling to secure blade to base
· Wood glue
You can make square sundials, but if you have a jigsaw or padsaw you can cut your wood into a semicircle. To make a semicircle, tap a nail into the centre of the longest side of your base, tie a pencil to a piece of string that is also tied to the nail, and draw the biggest arc you can. Cut with your jig/padsaw, and sand it smooth. Fix the blade in the centre of your semicircle, glueing the doweling down to hold it in position.
Put your soon-to-be sundial outside, ideally somewhere that gets the sun all day, maybe on a plinth, with the blade pointing north. The mystery of how to mark out the hours accurately is made significantly easier by the advent of personal mechanical and electronic timepieces. Over the next few days, visit the sundial on the hour every hour, from as early as you can make it to as late as you can remember, and draw the position of the shadow of the blade, not forgetting to mark which hour it is. Shadows are longer when the sun is low, so the middle hours of the day take up less space than the early morning and late afternoon, which oddly ties in with how they tend to feel to us. When you are sure it is accurate, paint or varnish it for longer life (for a surreal finish, on the blade and numbers, use luminous paint).