Right but when you start throwing rocks and glass bottles, when you start blocking ambulances and firetrucks on highways, well then you aren't a peaceful protestor anymore.
It is extremely hard to react with anything but EXTREME vitriol to comments like yours - police are using excessive force deliberately, and then arresting people with no cause who post videos of their crimes.
Yeah, that sucks. Your description makes it sound like their face was impacted by a metal canister, but looking at the picture, they seem to have simply been maced.
Without more context, I'm guessing their parents took them to a protest and acted illegally. If so, I think that's rather poor parenting.
There doesn't seem to be a link to the full video (or any video), so take that with a grain of salt.