All foods need to be given carefully when they’re that young. It’s usually best to introduce a new staple, ie veggie, one at a time over a few weeks. Just about any change can cause a change in their digestion.
I’ve actually never given my bunny oranges, but bananas are a favorite. My thought is that an orange is concentrated acid, while a banana provides a bit of potassium that’s good in very small doses. I would say limit to 1/4 inch wide slice of banana, maybe 2x a week max. You can also give a very tiny sliver of apple if you want to try another fruit.
Raisins make a good treat because they are the right size, dried and easy to digest. Again, no more than 2 or 3 at a time in a day or two.
But like with children, you don’t want to start them craving the sweets (fruits) young and spoil their desire for the good hay and greens. It is a treat for a little bunny to be played with a lot. Better that than food as a source of fun.