You do not need to offer the joey anything. She is too young to really be eating solid foods - and feeding her will reduce the amount of milk she will be able to consume because her stomach will be full. All she needs at her present age is milk.
Joeys will learn to eat solids by following the parents to the food dish - usually when they are 5 to 6 weeks OOP. The parents will introduce the joey to eating solids. When you begin to see the joey joining the parents at the food dish - increase the amount of all foods you offer for them to share.