Calibasco,
WOw, quite a challenge to explain twisted upon twisted logic to young minds. You'll need your magician's wand. But...my thoughts on this are that (a) you'll need a minimum of a week to go through historical grievances and chronologies. Without this context you would run the risk of trivializing the intensity of feelings involved; (b) try to avoid simplifying matters or reaching the all of nothing conclusion. If ETA were all nut cases, life would be easier. They are not, They put much wasted brainpower into creating an act meant to terrorize and kill and maim; (c) try to present strategic points of view, perhaps providing historical context from your home country (the US?). Of course, it is not the same, but getting kids to understand things from the perspectives of the actors involved would be an invaluable lesson for youngsters.

Your task is both tricky and challenging, but very noble. I wish you all the best.
A