I think that Non Working Days Different Colours is a very nice idea. I think it’s also a good way to get kids to work together, because they like the colours and they all want to be different. You can visit this https://masterbundles.com/templates/presentations/powerpoint/cute/ source to download cute templates for cute kids. It’s also a good way to get the kids in school to talk about what they are doing, which is important for learning.
If you apply the holiday to the “standard” calendar, it will display on the Gantt chart in the same colour as the weekends. However, it will only do this when there is a task – making the change to a blank plan doesn’t display the holiday; then when you add the 1st task, the holiday is displayed.