Rabbits need the long strands of actual hay - not chopped chaff. The long strands are needed so they can grind their teeth and to pass through their gut for good digestion.
Try normal hay-- meadow hay is fine if it is in long strands. Because they are so young, they can even have alfalfa hay mixed in too. Other grass hay to try could be timothy, oat, orchard, bermuda.
Some rabbits are particular about which type they like, but whatever type, it needs to be the whole strands of hay.