A s'more requires few ingredients: a marshmallow, a square of chocolate and 2 graham crackers to smoosh everything together.
Toasting the marshmallows is the main event (isn't that the most fun part, aside from the eating?). You can buy a s'mores kit (a mini hibachi fueled by Sterno) if the affair is going to be inside. Older kids and teens might like taking the party outdoors, toasting marshmallows over an outdoor fireplace in the backyard. Indoors or out, there should be plenty of adults on hand to help out.
Be sure to use long utensils such as bamboo skewers or fondue forks. Everyone prefers a different level of toasting but for best results hold the marshmallow a few inches from the flame and keep turning it until it's evenly browned. Try to avoid setting it on fire, as that's how accidents can happen.