1. Kid should not have been in the Street agreed
2. Kid should have been supervised. agreed
3. Rider should learn to ride a bike based on what info?
a. Rubber on the pavement can stop much shorter than the metal of the bike and the time it takes to Lay-it-down could have been used to slow or stop agreed
b. If no time to brake he could have swerved, it was a street, that means it is much wider than a person. you don't know how or where that kid came from, you're ASSUMING he had enough time to see the kid and react. He may have gotten spooked and just dumped it.
4. Vigilante justice does no good, in this case it is a good thing he was armed or maybe he would be dead.