Should I write a letter?

I am currently looking for a job. I really need the money because my mom is divorced and my dad has stopped sending us money, and my mom is not going to be able to support me and my brothers. I have started filling out applications at mcdonald's, burger king, etc... I was just wondering if it was prudent to write a letter to the manager of the site I am applying to, just explaining my situation and reinstating that I really need the job. What do you think?