I’ve generally found that they’re more interested in how you interact with the kids than the meat of what you teach. If the kids respond positively to you then you’re more than halfway there. Teaching them something they already know is hardly going to get their enthusiasm, but those doing the judging are more likely just going to want to see whether you can engage the kids.
The last demo I did (I got the job) for that age group was 90% over by the time I had done my intro, set the ground rules, got a few names and ascertained the general level of the class for myself. Make it clear to your potential employers in advance that you’re going to be doing this before you do any actual teaching, and you’re covered. Just go in, make friends with the little 'orrors, and spend 5 minutes at the end going over the lesson.
If you’re going to be doing a lot of demos then it may be worth developing a little ‘standard’ intro lesson that you can tack the learning objectives for the day onto.