What's the reason the children are given as to why Aunt Alexandra has come to stay with them? What do you think is the real reason?

Aunt Calpurnia cites the following reason for her visit.

"Well, your father and I decided it was time I came to stay with you for a while."

"For a while" in Maycomb meant anything from three days to thirty years. Jem and I exchanged glances.

"Jem’s growing up now and you are too," she said to me. "We decided that it would be best for you to have some feminine influence. It won’t be many years, Jean Louise, before you become interested in clothes and boys—"

In reality, I believe that Atticus knew how time consuming Tom's defense would be and how the community was going to react to him acting as Tom's defense.