One thesis you might explore is what was Mrs. Jones' motive for her actions. What was her motive for forcing (I think it fair to say "forcing, don't you?) Roger to wash his face? What was her motive for making him dinner (besides the obvious motive of feeding someone who was hungry)? What was her motive for telling him "were young once and I wanted things I could not get" without including the "but ..."? Most of all, what was her motive for giving him $10 to buy blue suede shoes (besides the obvious ones of generosity and compassion)?
There are probably a many different thesis statements to this excellent short story as there are people. What you need to do is think through what message you gained from the story and turn that into a thesis statement. The story concerns how one chance meeting managed to change the life of one boy, so you could construct a thesis statement based on how people influence us for the better, or something similar.