To whom is Aunt Chloe referring when she says that Haley’s master will be sending for him soon? What fate do Aunt Chloe and the children think Haley deserves? How do they feel about it? How does Uncle Tom react to this?

I believe Aunt Chloe is referring to God as the master, but Aunt Chloe, Jake, and Andy all believe he'll be going to "torment," that torment (hell) is what he deserves. When Tom arrives and speaks, he basically tells them not to wish that fate on anyone.

"Chil’en!" he said, "I’m afeard you don’t know what ye’re sayin’. Forever is a _dre’ful_ word, chil’en; it’s awful to think on ’t. You oughtenter wish that ar to any human crittur."