Aunt Alexandra wants Jem and Scout to act like a Finch.  She wants them to be just perfect and not to get into any trouble.  She says the finch name is a "Good Breed" of people.  Their dad goes along with it, but I think he doesn't really want his kids to be much different than who they are.  I think he is going to play some sort of trick.  Kids have to be kids and you can't expect them to be perfect while they are young.