Well, it depends on your goal. There is no one-size-fits-all measurement on this. It would entirely depend on what you want your participants to get out of participating.

Do you want them to learn how to code? If yes, then perhaps you can have a programming pre-test and post-test. See if they have indeed learned the language.

Do you want to know if they are satisfied with the content? Then, you can distribute a survey form and get their feedback.

There are still lots of measure that you can do. It all boils down on your goal. Hope this helps.

Thanks for response. I know that there are lots of measurement, but this event will take only one hour. Therefore, I can't have a programming pre and post test.

@Mehmet:

If the programming language you use is Scratch (https://scratch.mit.edu/hoc/) you could analyze the projects developed by the students with Dr. Scratch (http://drscratch.org/), a static code analyzer for Scratch projects that assigns a computational thinking score.

You can learn more about how to use Dr. Scratch for assessment here: https://www.researchgate.net/publication/281714025_Dr._Scratch_Automatic_Analysis_of_Scratch_Projects_to_Assess_and_Foster_Computational_Thinking

Good luck with your HoC activity. Best,

Jesús.

Article Dr. Scratch: Automatic Analysis of Scratch Projects to Asses...