The Red Coins on Tiny-Huge Island are located inside of a cave and require you to be on Huge Island. However, getting to the entrance on Huge Island alone will require you to do some bit of running around, although you have the opportunity for a bit of a shortcut. So, starting out on Huge Island, head through the doorway at the beginning and find the Koopa Troopa, as that is how you will be reaching the cave’s entrance without having to climb the mountain. Once you’ve found the Koopa Troopa, knock it out of its shell, then get on the shell.
You will be riding up the hill to the right side of the cave’s entrance, making sure to go up to the ledge with the tree. If you are up there and need to get off the shell, pressing the crouch button will make the shell disappear. Normally, you would have to drop down to this area from above, so this is a little shortcut. Once you’re on that ledge, head over to the wooden walkway and carefully make your way inside the cave. The cave is pretty small and all of the coins are easy to get, although some will require you to do Double/Triple Jumps. The star from the Red Coins will spawn in between the two pillars in the corner.