That barrier should also be sloped inward, or have a cap on the top that comes in, to prevent them from climbing over it. Yes, they climb. They also dig, so I have hardware cloth buried at the edges of my turtle pen, fairly deep, and then curled inward. That way, when they go to the wall and dig down, they hit the wire fencing, and can't escape.
I wish my hen-and-chicks would grow dense like that potted one in your photo.