Games. The first computers kids will start to wonder about are the ones they're using to play games, be it a game console, tablet, or PC. Modding comes first, slightly tweaking the game to add a new item, map, or remove some annoying limitation. The truly curious and persistent kids will eventually want to learn how to make their own games. This is where you'll find the next generation of programmers.
Definitely games. This is what got me first hacking when I was a kid. Now as a parent, my oldest daughter (now 13), has been hacking on things for a few years now. First it was with Legos, then Minecraft modding, and most recently she's been trying to hack Movie Star Planet. Believe me; I was quite shocked we she came to me out of the blue and was like, "Hey dad; can you come take a look at this? I'm having a problem…
I hope you had - either after or prior to that point - a heart to heart talk with your daughter about what she was doing.
It's one thing to do that on a game or whatnot - but we all know where that can lead, whether accidentally or deliberately, and we have all heard stories of people (usually guys - but I bet that's going to change in the future) doing something "for the lulz" or "because they were curious" or "just to see what would happen" - and then quickly finding themselves on the dark end of a TLA agent.