From the Super Mario Wiki
Sunflower Kids are a kind of flower found on Isle Delfino in Super Mario Sunshine. They lived outside Pinna Park peacefully, although their peace was disrupted when the Snooze-A-Koopas started nibbling on their roots. The Sunflower Kids turned brown and started to wilt.
Mario had to destroy each Snooze-A-Koopa to turn each Sunflower Kid back to its original healthy state. There is one giant sunflower called the Great Sunflower who is probably their leader. The Sunflower Kids, like many things in the Mario series, have eyes. Unlike the inanimate objects, though, they can move and talk.
Several Sunflower Kids make a small appearance in Mario Kart: Double Dash!!, where they can be seen happily dancing in Peach Beach after players beat a racing cup. Other sunflowers appear in the game; however, they don't look much like the Sunflower Kids, although they do have eyes, just like the kids.