They've replaced the cardboard cutout workers who had been mocked by a national newspaper because there were often more cardboard cutouts than actual workers. The replacement doesn't really address the issue, though, because if you read that sign and see no-one doing any work anywhere on the site, you're left with the impression that the child has been misinformed.
Oh well. The roadworks will probably be finished in the summer, only a year late, and the traffic congestion problems they've caused will be replaced by an entirely new set of traffic congestion problems.