Why does Atticus say thank you for my children to Arthur Radley?

Atticus Finch is thanking Arthur "Boo" Radley for saving the lives of his Children, James and Jean Louise. His children had been attacked by Bob Ewell , the father of a woman who claimed rape by a colored man, Tom Robinson. When actually her father Bob Ewell had beaten her because of her contact with Tom Robinson during an innocent interval at the Ewing home. Atticus Finch defended Tom Robinson against the charges of rape.

The Finch children are waling home in the dark from a school event. Bob Ewell is following them. He attacks both children. Arthur Radley has been following and watching the children in secret as their protector and guardian. Arthur uses Bob Ewell's knife against him to protect the Finch children. He then carries the injured Jem back to the Finch home while Scout follows him.