Depending on age my choices are:
From Earth to the Moon by Jules Verne
The Time Machine by H. G. Wells
And for the slightly older two books that are not pure science fiction, but have some connection to science fiction would be
1984 by George Orwell
Anthem by Ayn Rand
Finally, make sure they see the move (original) The Day The Earth Stood Still
The first two are pure science fiction, From Earth to the Moon foretold some actual events.
The other two books fall into the category of social science fiction .
The movie I suggested has science fiction elements but an excellent message.