You see Lucy knows what she has seen and experienced is real. Her brothers and sisters couldn’t see it with their own eyes yet, and even though they knew Lucy had never been one to make up stories or lie, they could not on faith accept what she said. But she knew it was real. C.S. Lewis puts it this way:
“For the next few days she was miserable. She could have made it up with the others at any moment if she could have brought herself to say that the whole thing was only a story made up for fun. But Lucy was a very truthful girl and she knew she was really in the right; and she could not bring herself to say this. The others who thought she was telling a lie, and a silly lie too, made her very unhappy…What made it worse was that these days ought to have been delightful.”(LWW –CH: 3).