mine love their greens - I give them a leaf or so (depends on how big the leaves are - they can be huge!)
If they haven't had them before though, only give a small square piece alternate days, gradually building up the size of the piece over a fortnight - otherwise you could upset thier tums as they won't have the necessary bacteria in great enough numbers to digest it. Depends how old they are, and if they've had much green veg before - greens are just young cabbage, so if they've had that it will be fine.