Today’s first glitch:
How it ought to be written:
A little girl is terrified of the monster under her bed, but what she doesn’t know is that it protects her from the true monsters: her parents.
Why it ought to be written that way:
There’s no need to say the monster under her bed twice in the same sentence. Replace the second one with it.
The dash after true monsters is used incorrectly because there is no abrupt change or break in thought. Replace it with a colon. (According to The Chicago Manual of Style, 16th edition, section 6.59, “A colon introduces an element or a series of elements illustrating or amplifying what has preceded the colon […] use a colon sparingly, however, and only to emphasize that the second clause illustrates or amplifies the first.”) If you don’t like the colon, just use a comma there, although this will give her parents less emphasis.