Secret Forest

The Secret Forest is a a special location where time does not matter. Merlin and the Three Good Fairies changed the flow of time there upon the request of Yen Sid, so it moves slower than on the outside world.

Settings and Areas
In this place a setting sun shines on vast forests and lush trees.

Kingdom Hearts 3D: Dream Drop Distance
Yen Sid mentions to Mickey that Merlin and the Three Good Fairies are aiding Lea in a place that is temporally flexible, so he can learn to wield his Keyblade.

Kingdom Hearts III
Under Yen Sid's orders, Kairi joins Lea and Merlin to become a Keyblade wielder herself. There she writes Sora a letter she never intends to send.

Trivia

 * While it's never revealed if the Secret Forest is its own world, part of a world or a different place altogether, its name is only used in the Theater Mode section and a loading screen of Kingdom Hearts III.