Scratch http://scratch.mit.edu/ is a programming language designed for kids. While it may be too early to learn scratch with a group of first graders (this may work out if you do it one on one), it is still useful for kids to see how programmers operate and what they think.
My presentation involved 20-30 minutes of programming real time. Of course I was very clear what I want to do and I tried it before, to avoid hiccups, which always happen but they are not needed when there is a bunch of kids waiting while you figure stuff out. My plan is to record a video of my presentation. For now here is the outcome:
Main concepts that were passed to kids during presentation:
1. Object oriented programming. Every visible object has costumes, makes sounds and can move
2. Objects can interact with each other
3. Coordinate system
4. Programming is a process of trial and error. Your hippo flies out of the screen - need to bounce it off the boundary. The rabbit is running too fast - need to slow it down, etc.