'Thank you for my children, Arthur.'

 When Atticus says this, he is thanking him for saving Jem and Scout.   Atticus is grateful to Mr. Radely for killing Mr. Ewell and is happy that he is not going to get into trouble.   .  The fact that Atticus who is usually very formal, even with his children, refers to Mr. Radley by his first name indicates that he is incredibly thankful to Arthur.  He is grateful to Mr. Radley for showing his children that there is some good still in the world.