Good guess, but no. The most salient difference in that case is that an abridged children's version of a novel acts as a direct market substitute for the origin

Good guess, but no. The most salient difference in that case is that an abridged children's version of a novel acts as a direct market substitute for the original, whereas a plot summary does not. (A secondary reason is that an abridged edition is likely to represent a much larger portion of the original work than would appear in a summary.) For further reading, see: https://en.wikipedia.org/wiki/Fair_use#U.S._fair_use_factors https://en.wikipedia.org/wiki/Fair_use#U.S._fair_use_factors