Rod, Genghis has some great advice in post #7 that you should really consider drawing from, as these are the sort of things I find successfu in my talks.
You don't say what age group the young people are. There's a huge difference in giving a talk to 9-11 year olds versus 14-16 year olds. Really think about your audience and tailor it to suit.
I find that a brief overview of history does the job - start at the beginning of 20th century and highlight how quickly aviation technology advanced in short bursts of time. Great examples are comparing 1938-1939 with 1944-1945, and then talking about what was developed during the Cold War - the 1950s and 1960s were a fascinating time for advancements in technology, and done with pencil and paper! But as Genghis said, don't bombard them with detail.
Props are great, and though your talk is about aviation don't be afraid to dip in to other engineering areas too. Motorsport is a great one to use because lots of kids are interested (or at least aware of) in F1 and there are a lot of similarities you can draw on from an education perspective.
If you know people working in cool jobs in aviation (or indeed other engineering areas) then drop that in too. A lot of children I meet don't really understand just what opportunities could be available to them if they consider an engineering career and when you start listing out what your old classmates are doing now (e.g. working on the F-35, testing engines at GE, helping to design next year's Mercedes F1 car, etc) then you can see the inspiration growing in their eyes right there in front of you and the questions start to roll in thick and fast!
I don't tend to talk much about a career as a pilot other than a brief overview because most children are already aware of it as a job. I do however talk more about flying as a hobby and I find that children tend to be more impressed by the fact that it's possible to fly for fun than actually becoming an airline pilot.
Good luck!