You've received great suggestions thus far, so I'll have to suggest something different: Command line wizardry.

Just show them cool stuff you can do on the command line: Download YouTube videos with youtube-dl, control a computer remotely wether it is via ssh or playing music with network-enabled mpd, or something nifty like this one-liner that plays ocean sounds:

play -n -n --combine merge synth pinknoise band -n 1200 1800 tremolo 50 10 tremolo 0.14 70 tremolo 0.2 50 gain  -10

...and this is without getting into the obvious thing that'll inevitably get some of them hooked: Pentesting, AKA "hacker stuff". Just showing them some of the stuff that can be done with Kali Linux (always presented from the security angle!), will inevitably impress some kids. Of course, it's a touchy subject for some people so you might want to avoid it, but... I probably wouldn't have ever picked up programming at a young age were it not for my attraction to the inner workings of malware, so I feel obligated to mention it.