My inclination would be to convey that it's fun to be a professional mathematician.

How many people in the world have a fun job that they love doing? Only a small percentage. I feel privileged to be in that group. If you can convey that, then I think that will be a more valuable message than any message about how mathematics is useful or noble or important. They'll hear those messages from other people. But they may not hear very often from someone who spends all their time doing math and feels lucky to have that opportunity.

In terms of actual mathematical content, I like mathematical games and puzzles myself, so I personally would try that tack. Kids who have won a math competition are probably going to enjoy something along those lines. But the main thing is to pick a topic that excites you personally so that your enthusiasm will be obvious.