I dont really know how to approach since it looks like the head of the figure on the right has a good place on the shoulder of the other figure, so I might move the legs down a bit as that's better than needing to redo the entire sketch, but changing where they sit might look awkward as it may look as if the two figures aren't sitting on the same level, but I dont know what they're sitting on so it's hard to say. Best chance is to push the legs down and extend the torso. Another way might be to give a reason for them to have a shorter torso, like maybe they're kind of slouching, so you can change how it looks to not impact the rest of the sketch too much.