How to remove background hiss?

Well, I answered my own question.

You cannot do this within iMovie. Instead you have to export the audio and then run it through another application which is what I did using Audacity. I exported the audio to a .wav file and imported into Audacity. With Audacity, you can sample a bit of audio where no one is speaking using the "Remove Noise..." filter and then apply that filter to a selected section of audio. There's lots of other things you can do with Audacity but all I needed was removing background his and amplifying a few other sections of audio where kids were speaking to softly. It worked great! Once finished with Audacity, I exported from Audacity back to a .wav file format. Then I imported that hiss-free audio track into iTunes which then made it available in iMovie. Before dragging the full hiss-free audio clip into my movie project I first muted the existing full-of-hiss audio track. Because the audio tracks were the same length, it all lined up perfectly.