What two ways does Aslan appear to the children and explain the circumstances and purpose of his contact with him ?

Aslan appears to Lucy in the pages of a book..... a book of spells. When Lucy turns, Aslan is standing in the doorway, correcting her for using spells to eavesdrop on her friends.

Aslan later appears to the children in his own country..... he appears in the form of a lamb. He is there to tell the children that that have to go home.... that they will not be able to return because they are getting too old.