Fasting is a wonderful idea...for adults. We have the discipline and fortitude to fast from food, watching sports, or fiddling with gadgets because we're mature enough to handle the self-denial aspects. We understand that setting down a phone for an entire day or a week is wise and makes sense because we can always get back to email the next day. We understand adult concepts like delayed gratification.
With kids, it doesn't work quite the same way.
One of the issues with putting younger kids or even younger teens on a fast of any kind is that they don't really understand the variables and they don't understand how an entire day of fasting can lead to a smarter approach to consumption. They barely understand consumption. You would never ask a kid to stop eating for ten hours, mostly because it's unhealthy for them but also because they don't really need that level of discipline.
You can decide to stop serving them pizza for dinner every night, and they will probably comprehend why that makes sense. Too much pizza is a bad thing. Kids are at the stage where they can understand why eating ice cream all day is unwise.
Kids also don't comprehend the benefits of a digital fast, however. The problem is that they barely understand the gadgets, let alone the science of dopamine or why the shiny devices are so compelling. All they know is the gadget can show them Peppa Pig videos.
I know this from personal experience. With my kids, I used to "borrow" their phones for short stints. It was more about disciplining them than teaching them the concept of self-discipline, and I'd sometimes make a joke about it. "Hey, can I borrow your phone?" I'd say. They would groan and hand the little plastic gizmo over. At times, I'd teach them about self-discipline and making wise choices in life. I never asked them to fast.
I know some parents who decide not to let their young kids use gadgets at all. Apparently, Steve Jobs didn't. Bill Gates also limited screen time. I support that, and it makes sense.
My issue is with digital fasting, a concept that is not meant for kids. They don't really "get it" in the same way that an adult understands self-denial and discipline. We're mature enough to handle a digital fast because we can work out all of the complexities--letting coworkers know we won't respond for a day, using an automatic reply, or preparing for the fast in other ways. We're operating at a higher level than kids.
How do kids perceive a digital fast? Well, for starters--they don't. They don't understand the layers of denial, discipline, delayed gratification, or throttled usage. To kids, you might decide not to let kids use a device at all, but explaining it in adult terms doesn't make sense. It doesn't teach them the principles of fasting because they are too young.
My advice is to use a totally different approach that has nothing to do with fasting.
Don't bother talking about denial, fasting, or discipline.
What works is limiting their exposure and allowing use in short intervals. Decide to let younger kids use a device for short periods of time, say 30 minutes or less. Keep the focus on how much you allow kids to use a gadget, not how long you are preventing them from using it. Over the holidays, enforce that rule and be clear about it, but skip the part where you try to explain how it's for their own good or that it teaches them discipline.
It will, in the long run--eventually they will understand digital fasting
For now, view digital fasting in the same way you view fasting from food.
In other words, don't use that strategy with kids at all.