sectism izm skizm

i teach 13-15 year olds at two different weekend islamic schools. recently, kids at both schools asked me "what are the shia?" and "what is the difference between sunni and shia?" how to respond... before i respond to the kids, i need to be clear in my own understanding of the evolution of my muslim community. my own subject is architecture and i've read much history to understand its context, but i havn't studied sociology and anthropology, and i only skimmed through a translation of ibn khaldun's muqadima, i know there is more to it than what i write below. but, here's how i see it... two aspects: componants and interactions between them... aspect#1- understanding the individual components that make up the muslim community. (my design oriented mind thinks graphically, so in my notebook i've drawn diagrams -a circle to represent the componant and a point in the center to represent its authority figure) here are the components: listed as COMPONANT (authority figure) -1. PROPHET'S LIFETIME UMMA (prophet) -2. RASHIDUN PERIOD UMMA (caliph) -3. POST RASHIDUN TRANSITION rival CALIPHATES and SHIA revolutionaries -4. ABBASID PERIOD UMMA (ahl us SUNNA caliph, sufi shaykh, SHIA imam) -5. LATE ABBASID PERIOD (sultan) -6. MONGOL CONQUEST SULTANTES -7. EMPIRES (badshah) -8. PRE MODERN PERIOD (colonial governor) -9. POST CALIPHATE NATIONS (revolutionary leader)... aspect #2: how these components and branches within components relate to one another and interact with each other is more complicated. (graphically i have the circles that i described above overlapping each other in various ways during the different periods of our history.) ...of course many times the complicated social dynamics and interactions between individual human beings that make upour muslim community cannot be reduced into neat bubbles. if it were only that easy... ex. one of the girls in the class has a shia father and sunni mother.... so, knowing what i know about our history, what should i say to the 14 year old child that asks me "whats the difference betwen shia and sunni?" how much history do i share with them at that stage of there growth? and what means of communication should i use? my first instinct is to tell them a story of the key incidents that caused the split. then give them the very very basic difference between the two like i described above: shia placing the imam above all and the imam being a decendant of ali. i want to come up with a role playing exercize with problems similar to the ones that caused the spilt. and have them act out what they come up with as a solution, then compare their solution to the shia and sunni response. is this too much for them at that age? should i stop at the story? i don't know what to do. help.... peace and love.