The fictional land the children enter in C.S. Lewis’s Narnia books is Narnia.
Narnia is the central setting of The Chronicles of Narnia, a series of seven fantasy novels published between 1950 and 1956. The world contains talking animals, mythical creatures, kings and queens, and a recurring struggle between good and evil. The stories connect Narnia with other worlds, including Earth and Charn.
Aslan, the great lion, is Narnia’s most important recurring figure and serves as its creator and guardian. The Pevensie children reach Narnia by different routes, including a wardrobe, a magic ring, and a painting. Their visits occur at different points in Narnian history, so time does not pass at the same rate in both worlds.
Narnia is also the name of the country rather than the title of the complete series. The full series title is The Chronicles of Narnia, while individual books focus on particular adventures within that world.