The song "This Time Tomorrow". Tom has just testified against Joe, and is understandably frightened about reprisal. In the middle of this, Aunt Polly, usually at odds with her nephew, sits next to him and sings a song about how fast he's growing up and how she will always love him.
The song "I Can Read." Huck is just so excited over his accomplishment. In addition, part of it goes, "I can read, and I can write and they were wrong" — suggesting that some people have told him he was too dumb to do it.