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

Alexandra has insisted on coming to help with things while Atticus is busy with the Tom Robinson trial. Really, Calpurnia can handle domestic duties but Atticus concedes because she is his sister. Although Atticus tells the children she is coming to "help", the kids know Atticus is relenting because of her forceful personality,

"We felt it was time you children needed--well, it's like this, Scout... Your aunt's doing me a favor as well as you all. I can't stay here all day with you, and the summer's going to be a hot one." (Chapter 13)