All right, maybe I used the term "kids" where I meant "youths". We Germans often do when talking in our mother tongue, because the word "Jugendlicher" for youth is inconveniently long. Sorry about that.
Concerning the 10year-olds present: they were most likely taken there by parents or elder siblings. But why shouldn't they? Remember, we are talking about a park in the midst of a 600,000-people city, the most important patch of green here. Should we keep our kids away from that just because the first use of watercannons against people in the history of our town might happen? You might reply that they should have moved away as soon as the police started to clear the scene. Well, several people told me that they had no chance to get away in time.