I will admit to not following the details of the incident very closely, but you make an important distinction as to precise timing of Wilson's awareness of the strongarm robbery.
The account presented here is the most detailed one I've seen. My vague impression has been that the police made a point of saying Wilson didn't know about the robbery (period). I'm not sure that's what they actually said, or if it was qualified as "robbery wasn't the reason to tell them to get out of the middle of the street - although that's sort of a "duh, no kidding" point. If the initial request to brown was to get out of the middle of the street, it seems rather obvious that Wilson didn't suspect Brown of anything other than walking in the middle of the street.
The police chief was asked repeatedly if Wilson knew of the robbery. He said that he didn’t know. I think the narrative suggusts that he didn’t stop the two kids because he knew of the robbery. There is some indication from the narrative that Wilson some time later, within a minute or two may have heard on the scanner that there was a robbery.
One thing for SURE, Brown was aware of the robbery. He had just committed it. He didn’t know if Wilson knew.