Facebook is not the only cool thing kids know about.
Your PR guys know their trade, you should definitely use something that kids consider interesting as an example. However, what they don't know is how much complexity a service like Facebook is about. If you were to even start to talk about all that you'd probably bore them to death.
Try to choose something as simple as possible. A Facebook app is probably a good choice if you manage to avoid explaining too much about the platform itself and focus on the app instead.
I don't know what does your company make, but also consider other things potentially interesting (to kids) but relatively simple enough to be explained:
- mobile apps and games
- anything with geolocation
- augmented reality or something with image recognition (ok, this one is probably difficult to explain, but it does look cool)
Whatever you choose be sure you include lots of screenshots with something cool on them and it would be best (if feasible) to include a live demonstration of the app in front of them.
I recently read in newspaper how a new physics teacher in some highschool in my city started the very first class by bringing an actual robot to the class (one of these Japanese robots we see dancing in commercials, I have no idea how he got one). His point was to show these kids how physics is used for cool stuff, not just boring calculations. Do something similar (with software, not robots).