Those are not Bernie’s kids, those are Hillary’s kids. Bernie is getting the blame for something Hillary/Soros is responsible for, just as the “Trump’s kids” at Bernie’s rally, are also Hillary’s kids.
The major reason Hillary’s campaign is doing this is to attempt to prevent Bernie supporters from crossing over to Trump. And, of course, to trash Trump’s image with the voters.
Well.. in Chicago, if they were Hillary's kids, they somehow found the need to hoist "Bernie 2016" signs.