In Chapter 19 of To Kill A Mockingbird, readers learn, through Tom's testimony, that all of Mayella's siblings were in town getting ice cream at the time of the supposed attack.
During Atticus's questioning, Tom tells the court that when he questioned Mayella about her siblings' absence, Mayella told him,
It took me a slap year to save seb'm nickels, but I done it. They all gone to town.
This information is significant because it shows us that Mayella is the primary caretaker of her siblings (we know that she keeps a garden despite the fact that the family lives behind the town dump) and that she cares for them enough to save all the money she got for their benefit. Further, this plot to send the kids to town was probably an attempt for Mayella to get time alone with Tom. Ultimately, readers see Mayella as a lonely character who falsely accuses Tom Robinson of rape in order to cover up the fact that she made advances toward him.
Mayella never answers Atticus when he inquires about the location of her many siblings during the "attack" she supposedly suffered. Atticus's cross-examination had created a pitiful picture of the Ewell lifestyle, one in which the kids didn't go to school because having two people in the family who could read and write was sufficient, the father probably drank up most of the family's relief check, and "dumpster diving" in the nearby town dump was the primary means of sustenance, other than what may or may not have been left of the relief check after Ewell signed for it. It is apparent from the cross-examination that there is nowhere the siblings would likely have been during the "attack" that they couldn't have heard Mayella scream; therefore, Atticus seems to want the jury to infer that Mayella had sent the children somewhere ahead of time. Of course, the train wreck that is Mayella's testimony will be completely irrelevant despite Atticus's best efforts, because Tom Robinson is still black, and as ignorant as Mayella and her father are, they are still white.