I'm in healthcare, and often work with children. It doesn't matter if they do or don't understand something, they don't have the emotional regulation to continue with something uncomfortable.
I don't blame parents wanting to increase the sleepiness of a child prior to something like a long plane trip.
8+ is the age I see the majority of children being able to grin and bear it, and to be reasoned with.
Sounds like it shouldn't be considered legal for onroad use there. :/