I have done several demos for jr high kids that lasted for about 15 minutes before the groups rotated to an other site (it was part of a historical musem so there were other demos) What I did was some simple like leaves and did what Steve talked about, breaking the project into steps and have the progression made up ahead of time so while you formed the point you have a point in the forge, so you can move from one step to the next with out having to wait for the metal to heat. This way you can make a leave in about 10 minutes while explaining your steps. Then you can do fun things the impress like brand your stump to show how hot the metal is and other things the kids are interested in. I usually had made ahead of time several leaf coat hooks so each teacher would have one to put up in the class room reminding the kids what they saw demonstrated.