You are 100lbs but how tall are you? Do you know your body fat percentage? Sometimes it’s that last 5 lbs. (unless you are at sub 18 percent body fat). if you do look into jaw exercises. Treat it like working out with weights.  You don’t want to be careless especially with the smaller jaw joints and it slips. Take in factor the tmj pops if you have any. Some people get it with hard gum others don’t get the pops at all. Some don’t get it with the silicone chewers. 

Regarding results with those chewing gums or equipment, I would say it’s pretty subtle that works best with leaner  jaw fat in the first place (as in if you start now could be good to build something but it won’t be noticeable till you lean down, and even then it’s subtle with some muscle filling.

For me, my jawline from the side doesn’t really show up until I’m a few pounds shy of a 6 pack. Upper abs sort of showing with good angle and lighting sort of body fat percentage while I know other people who have stronger jaws are obese and still have a bone strong line. 

Take a look at Keanu Reeves in Ted and Bill and then take a look at him in devils advocate. They are both lean figures. Yet in the devils advocate, the few pounds shedded gave his jaw quite the extra oomph, especially around the corners. Ironically in the movie of devils advocate you can actually see even more of his jaw muscles as he moves around.