Stopping the kid from riding through the concrete was completely justified, whatever was going on in his head. Maybe he saw the cones, knew there was fresh concrete and meant to mess it up. Or, maybe he was just riding heck-for-leather and never noticed the cones at all. Given my experience of boys, either scenario is believable. Doesn't matter -- stopping the kid was necessary and appropriate. And, given the speed with which scooter-driving 11-year olds move, shouting at him was the best way to achieve this.
My only quibble is with the word 'moron.' That particular verbiage seems harsh and unnecessary. And it could, depending on how protective or over-protective the kid's parents are, cause problems with the neighbors. Myself, it wouldn't have concerned me to have my kid called a moron (assuming, as in this case, that he was behaving stupidly). But, as you can see, some parents would be more bothered by it than I would be. A better choice would have been to say, "Hey! Don't you see the cones? They mean there is wet concrete here. Go around!"
I wonder about the 7 year old girl, too. I doubt very much that either of my kids would have been writing vulgarities in concrete at seven. Older, maybe, but not at seven. My guess would be that an older child did the writing and pinned it on the little one.