What code requirements exist for low-sill, upper-floor windows?
Windows above 72 inches from exterior grade are required to have a sill at least 24 inches off the floor to meet 2012 IRC requirements, which should be what you're required to comply with (IRC at least, it might not be 2012 but AFAIK it's been 24 inches for years). If you can limit the windows from opening more than 4 inches, or use a "Window Opening Control Device" that limits it to 4 inches, but releases with more pressure (no more than 15 pounds, however), then they are OK below the 24 inches. Here is a link to the code. 

If you prevent the bottom sash from opening at all, and the top of the bottom sash is no less than 24 inches from the floor, then it meets the requirement. Your closet window needs to meet this requirement as well (meaning, allowing it to open freely from two inches above the floor is not OK). Or, you can install guards (I'm guessing that the guards would have to prevent a 4" sphere from passing as well). 

Recommendations from my own experience would be to replace the glass with tempered if you are able (I'm surprised it's not required), just in case a window breaks there will be less of a chance to get cut. And, if you're expecting to have rambunctious kids (or dogs or whatever) in the house, I would install guards even if the windows don't open. Just in case.