To get to this world you need to beat the boss of World 5-Castle as Mini Mario.
World 7-1
Start off by heading right and riding on the moving platforms. Let the second platform fall and head left. Jump to grab the first Star Coin. Ride the platform back to the right and jump off before it falls. For the second Star Coin, you must hurry. After the platforms, there are two ?-blocks and then a hanging wall. To the right of that is flying ?-block. Jump on it and let it carry you to the right. At the end, get off of it onto a cloud. Head up through the green warp pipe. Swing yourself between the moving poles to reach the second Star Coin. Head down the green warp pipe on the right. Unfortunately, this coin made you skip the checkpoint, so you cannot die. Go right and up the green warp pipe. Hit the flagpole to finish the level. For the third Star Coin, you must do the level again. Go back to where you jumped onto the flying ?-block, but don't get on it. Keep going right and through the green warp pipe at the end. Head right and get on the top moving platform. Grab the third Star Coin, and finish the level like your first time.