5 cup sugar
2 cup water
Food coloring
Aluminum tins (disposable)
String or thread
Candy thermometer (must go to at least 325°f)
Wooden spoon

Buy some disposable aluminum tins - square or loaf seem to work the best.  Use a pencil to punch small holes into the rim of the tin and drape as many strings across the pan as possible without any two strings touching.  Tie knots in the string outside the rim holes to secure the hanging string.  The mixture you're going to be pouring is hot enough that any tape used to secure the strings might come off.

In a medium saucepan bring the water to a gentle boil.  Add the sugar and stir with a wooden spoon.  Continue to boil until all the sugar has completely dissolved.  Food coloring (1-2 drops) may be added.  Continue to boil until the temperature reaches above 260°f then remove from the heat.  Let the solution cool for about 5 minutes then pour into the prepared tins.  Cover loosely with aluminum foil or waxed paper (just to keep dust off; do not use clear plastic wrap).  Punch holes in the cover to allow evaporated water to escape.  Place in a warm, ventilated space - like a window sill.  Takes about a week or less for all crystals to form. 

Different colors: one or two drops of food coloring can be added to each tin after the solution has been poured.  You can make different tins with different colored crystals.

Candy sticks: use wooden coffee stirs or popsicle sticks in one tray.  Kids can use these sticks at home to sweeten iced tea. 

Seed crystals: drop a few "seed" crystals of sugar into one tin without any strings or sticks.  See if crystals form.

Questions: ask kids if the patterns of the crystals look like anything else they may already know...  Buildings, rocks, jewelry, art...  Have them try to draw their crystals and discuss with others.  (did you know there's a national park called "Rock Candy Canyon." Why do you think that is?)

Rock candy, like any candy recipe, is very unforgiving and can fail.  I recommend making it at home first to be sure you've gotten the steps down and can reliably create a working supersaturated sugar syrup.  Have some rock candy stirring sticks already tucked away when the magic day comes to pull the strings out of the solution to ease failure.

Crystallization can occur on surfaces other than your string or stick.  If you're string is lacking in crystals, pour out the water and examine the tray.

Kids find it hard not to touch the apparatus.  You may want to shield it from their view during the week or limit access to 5 minutes a day.

The more rough the thread or string is the better crystallization will occur.  Very smooth thread - or heaven forbid - fishing line! Doesn't form good crystals and may not work at all!