I learned a lot working with a class of 5th graders for 6 months, researching and designing a mobile tourism application for my Master's thesis.
I liked explaining User Experience in it's broadest sense (not just limiting it to interfaces). I found you can get quicker understanding from children if you explain it using items in everyday life, or services. A lot of design for children isn't as much about things being useful and necessary as much as it's about them being fun and age-appropriate. I used an example of games: chutes and ladders vs. scrabble. Maybe showing a fun, colorful (and age-appropriate interface) vs. a "boring" one could work too. Or a phone that fits in their smaller hands as opposed to a big galaxy note for example. After all, UX is about first impressions, desirability, and usability too!I also found that open-ended questions don't work well with such young users. They have a tendency to please, so they will say they like everything and they can't explain to you why they like something or why they did what they did (when using the interface). These problems are well noted in research into interaction design and participatory design with children (check out articles by Allison Druin for more information).
Kids can really amaze you with coming up with new ideas and criticisms, but you have to invest more time into it. Talking to them one-on-one, more involved questioning/ interviewing, letting them draw their ideas, etc. I had better results from more in-depth interactions myself, like presenting cartoon storyboards and asking what they think the characters are feeling using the application.
In a presentation, your interaction with them can is limited to a large group setting so I wouldn't count on them voicing their opinions to the crowd and being constructive or critical out loud in order to participate in your presentation. It's always important to research your audience first (what are the cognitive abilities and popular activities for these age-groups now-a-days?). After that, I would go for simple, fun, and funny (maybe even silly?) :)
If you've presented UX to kids at a career day or otherwise, please share what worked well for you!
Photo credit: Margarita Lyukmanova for the TrenTour Project. All rights reserved, 2013.