I have a kindergarten under the sea

Serena wakes up as an F-class princess of the Shell Clan—betrayed by her sister Lilith and her ex-fiancés, and dumped on a sunken ship. There she unlocks a Kindergarten System.

As she cares for Eldrin—now a child—and the other sea lords, she slowly wins their hearts, and ends up saving the ocean itself, becoming its queen.