Or at least that’s what we’re told in Prince Caspian. The truth is, however, slightly different. It is true that for the third story only Edmund and Lucy return. All four return for A Horse and His Boy and three of them return for The Last Battle.
What I believe Aslan is trying to say is that the two older ones have learned all they are going to for now, and the next adventure will be for the younger. (I’m actively reading the books now with the little ones so I’ll let you know…)