Types of litter you can use are: scoopable kitty litter (preferred by most rabbits), wood shavings, paper litter, and potting soil so you can throw their manure straight into the garden.
NOTE: Baby bunnies are babies for the next couple of months. They can’t ‘Hold It’ like the adults’ rabbits. Give them frequent potty breaks; they will have to go after a nap on you or in your pocket and when they are eating.
Have the baby bunny’s food bowls where they can eat and poop at the same time (adult rabbits don’t eat and poop at the same time but the babies definitely do).