i mostly give mine kale, sometimes other greens as well (beet or radish tops, lettuce, etc) -- and they have similarly learned to stand on the kale leaf & rip off a bite-sized piece. although if i have time i'll usually stand there with the greens held tightly in my hand while they peck at them, just to make it easy at first.
i once gave them a head of green cabbage, and the outer leaves were too tough for them to get through! but they were still teenagers then, i should try it again.
Yes, they get better at ripping up things when the get older.
It is almost time for Pumpkins. I cut a hole in the side of the pumpkin and they will eat the inside of the pumpkin from the hole. Very cool to see.
Laura, I am very excited that you are hatching Duckwing Marans. I can't wait to see them.