I did a peanut butter jelly robot - where I had the kids call out how to make a peanut butter and jelly sandwich. They had 10 minutes to write out the instructions. I took each instruction as literally as possible. Then I talked about algorithm design. It took about 30 minutes.
I'd suggest not trying to impress them. It's not a date. They're kids. Try to connect with them. Talk about your job, sure, but more important to share things you're enthusiastic about.
Also, from what you describe of their background, they may think that what you do is inaccessible to them. Try to reach them, especially the girls. And let them ask you questions.