I agree with Syberchick. The kits are likely mix-breed rabbits, and they look very young. Not all rabbits or all kits will have a bad reaction to fresh produce, but when it does happen it can kill them very quickly.
Most long term rabbit owners have learned to be very cautious with kits and only feed them the foods that the mother ate while pregnant and the kits started eating. It's because their ability to digest food comes from delicately balanced GI bacteria which is inherited from their mother.
Without their mother's acclimated digestive bacteria, the kits will have to slowly develop their own. Consequently, survival rates are much higher when people wait until later to introduce unaccustomed kits to new foods.
I feed de-stringed celery to my adult rabbits, but the only greens I feed to kits are in the "ultra safe" category, plants that are more likely to cure diahrea than to cause it.
Unsprayed rose, blackberry, raspberry, or strawberry leaves and plantains (greater and english or lance leaf) are among the safest. Willow and apple twigs are also very good and unlikely to cause a GI imbalance. Anything high in sugars or starches should be avoided.
Welcome to the world of rabbit raising
Those cute little guys can be deceptively complicated