They should be able to handle hoppers right away. A lot of breeders feed small adult mice to hatchling bps. Pinkie mice are really too small for most bps. Do you have weights of the babies? As a general rule, I usually feed prey items that are about 10% of the weight of the snake. Another method to choose prey size is to choose a rodent that is about the same diameter as the snake at its thickest point.
Steve