Excuse me for bursting your bubble, but 'proper training' includes knowing what you shoot at, and hitting what you shoot, and before all of that...not using your weapon lightly. It's been pointed out, numerous times, that no one is sure if the kid that was killed was even throwing rocks. Any way you look at it, the training was either inadequate or the officer failed to follow it.
Which complicates the nice picture of him going home to his family in safety, when his job is now in question, criminal charges are possible, and god forbid, he has to hug his own 14-15 year old child and think of how his errantly fired round killed one.
The shield can be easily carried on the back of a bike. Just like a child seat can. Either way, I doubt they were riding the bikes around, under the bridge, in the mud. Which means they had to dismount. An extra few seconds to snap a shield off the end of the bike, knowing that they were going into an area where rock attacks have increased dramatically, is common sense.