Why do the children's attitudes change towards him?
Remember the description of Boo Radley at the end of chapter 1- long jagged scar, malevolent phantom etc.
Is it because he becomes more real to them?
Scout's change of attitude is described near the beginning of Chapter 26. She realizes that the children's games "must have been sheer torment for Arthur Radley," and she feels ashamed. But she remembers also the gifts that were left for them in the tree. Finally, she fantasizes about how she might some day see Boo on the porch, and greet him with, "Hidy do, Mr. Arthur." This shows that with time, she has outgrown her picture of a hideous monster lurking about at night.
Jem's viewpoint has changed, too; walking to the Halloween party he laughs about the silly games they plade only a few years before, as they walk past the Radley place.
It is at the very end of the novel, however, that Scout is able to give voice to her understanding, as she stands on the Radley porch and realizes that Mr. Arthur has been latching and caring for "his" children.
Partially it is because he becomes more real for him. The children's attitude towards Boo Radley change for two reasons. One, the children simply mature in their understanding of human nature. For example, Jem learns by reading to the old lady who is trying to come off of morphine that people are often not what you think they are in the beginning. Through the whole trial the children also learn that the people who are supposed to be 'good' are sometimes not so good. So the children mature during the story.
Additionally, Boo has been leaving toys for the children in the tree stump and as the lives of the children are threatened, Boo Radley kills Bob Ewell who is trying to harm the children. They realize then that Boo is not something scary and bad but a person, who although his ways are different from most peoples, is still a good person. They learn that differences do not make someone bad.