Even if the kid didn't get shot, he probably wouldn't have done anything. He probably thought it was badass to witness a train robbery like any other kid. Wouldn't be surprised if Todd was an undercover cop or someshit with a license to kill in order to take down the blue meth operation, but I doubt that's the case.
he had just arrived after the train left; walt was facing that way the whole time and only saw him at the last minute. he didn't see shit except three men celebrating.
also the wire and breaking bad have very different goals. the better comparison would be the sopranos, but even that is too different. breaking bad is its own thing.