What page is the quote Thank you for children Arthur in To Kill a Mockingbird?

The quote "Thank you for my children, Arthur" does not appear in Harper Lee's "To Kill a Mockingbird." It is a misattribution.

There is a similar quote in the book, but it is spoken by Boo Radley to Scout after saving her and Jem from Bob Ewell:

"'I couldn't let her hurt you. I wouldn't let her hurt you. You're real nice people.'"

It is possible that the quote you are thinking of is a misremembered version of this line.