It is possible he was taking a late round in the hope of making more money. It would be unusual for the van to play music just by starting the engine. He may also have been running late due to traffic but then why play music?
Just because it would be a stupid way to kidnap anyone doesn't mean it is not a threat, many criminals are thick. When it comes to children there can be no risks, I would suggest reporting it to the local police so that they can keep an eye out and also maybe your local neighborhood watch, particularly if this is a frequent event.