Try bending down and tapping on the edge of the bowl or plate with your fingernail, imitating the way they peck at things they're curious about. I do that with a lot of new things for the chickens and my parrots. I also run my hands over things or grip them, as they watch me, when they're afraid of something new. You could also take one of the bowls or plates with your own food on it out there and eat a few bites from it, as they watch. You could also put a food they recognize and enjoy in it, like scratch. That can often tempt them to be bolder.
Usually it only takes a couple of minutes to teach chickens about new things. The more you do it, the easier it gets. They'll learn to trust you and look to you, when they're uncertain about something. Parrots are the same way. The best way to get them to eat something new is to eat it in front of them. The best way to show them something new isn't dangerous is to handle it in front of them.
