You'll get the "how do I hack?" "how do I make games?" questions no matter what. But if you do the talk about right, those will be flippant jokes rather than serious questions.
Basically you need to open with your way of saying "everything you've seen on TV or in movies is wrong. There are no falling columns of Matrix code controlling everything, and there is no 'hacking' by flying through 3D cities or typing for 30 seconds. World of Warcraft took sixty million dollars and three years to build. Whoever fried Iran's uranium centrifuges, wink wink, took years of planning."
You don't really have time to go in depth, nor do they have the background for you to show them actual code. So don't worry about those. The key is to pick examples that they'll already be at least a little familiar with and that you're comfortable with, and realistically de-magic those examples a bit.
I'd recommend two flavors of examples before you open for questions. The first one is based around "this is what I do in real life". You can very easily tie your database stuff to, well, every big popular site the students have ever used. Facebook, Google search and maps, any webmail, ebay, Amazon, itunes, 4chan, Slashdot, and so on. All of that is based on gathering, sorting, storing, and searching through vast amounts of information. You can do the old dictionary example - use a physical dictionary, solicit a word from the crowd, and then look it up quickly right in front of them. Then point out how it'd take all day if you had to read every line in order from the front or the back. Then point out that Google's database printed out as dictionaries wouldn't fit in the entire internal volume of the school - floor to ceiling, wall to wall, all the rooms and hallways and the cafeteria and gym and auditorium and so on - and yet Google needs to do millions of lookups per second. All that is math. Not Einstein's rocket ship time machine math, but stuff not much harder than what they'll see next year in Precalculus. But without the math, it's like the warehouse at the end of Indiana Jones, where cool things go to die (because no one can ever find them again).
The second one would be any common-but-hard problem in games. Something like pathing AI. You don't need to have actually written those programs; the idea is that you can explain it's too complex to calculate every possible path and pick the best one. The gamers in the crowd will grasp this fairly well, because they've all seen games where the pathing sucked, or where the third person viewpoint kept blocking them, or where the AI enemies seemed to cheat, or where the interface was poorly designed and you could never find what you needed in its maze of nested sub-menus. Again, it's all math. But it's hard-but-interesting math. The computer can do the calculations, but you need to know what its limits are, what calculations need to be done to do the work you want, and how to tell the computer to do those. You cannot say "computer, make me a sandwich"; you've got to write it a cookbook first.