@elena I love explaining stuff to small children!
If I would have to explain (some aspect) of digital sovereignty to a 4 year old. I would do it as follows: in your class, you have toys. Which is nice, so you can play. But in some schools, there are also toys that are shared over all classrooms. Is this the case in your school? Imagine that you your class has shared all its toys. But now the other classes have decided you can't play with them anymore. That would be no fun and not fair.