It would depend on what he looked like and if my family was there. If he didn't look armed I would confront him. Otherwise just discreetly call the cops.
I'm not afraid to do the right thing, but I'm not the kind of guy who brings a knife to a gun fight...especially over a bicycle.
But to give all those people some benefit of doubt...the "thief" was so blatant that it created confusion amongst the onlookers.