Sentence:  While a faint whiff of raffishness remained, science fiction could be justified as a specialized form of children's or Young Adult literature—as literature <WSD>with training wheels</WSD> on it, something that would teach the kids to be familiar with Science and Technology and to think progressively about The Future (much in the fashion of the famous “World of the Future” dioramas at the 1939 World's Fair) until they were ready to put aside childish things and turn to “real literature” about “the real world". 
Meaning of the word marked with <WSD> tags:  Designed or adapted for children.