The minis start on a span of ground right in front of the locked Goal Door. However, the Key Mini Mario stands behind the keyless Mini Mario and cannot initially reach the door to unlock it. After both toys turn away from the door, they need to be cross a pit of spikes with some Pink Blocks and reach the top left platform, fall through a tight gap back onto the lower ground, turn the other direction, then cross the spikes again using Pink Blocks. This will put the Key Mini Mario in front, allowing it to reach the door first and unlock it to clear the course.
Mini Mario Card
The Mini Mario Card is on a platform in the air. From the bottom left side of the stage, the minis should be led to the card with a Pink Block staircase so they can reach and collect it.