Pg. 55This is the scene in the book that takes place at the start of the summer in which Dill tells Scout that he loves her. This shows their innocence obviously because both are still very young and still haven't experienced actual love.
Pg. 55Shortly after he asked her to marry him and Scouts lack of care/awareness of the actual situation just demonstrates her childlike mind/understanding.
Pg. 55Lastly then Scout goes on to say that Dill had said, "I was the the only girl he would ever love." Referencing back to past things that were said it just adds along to the fact that they are still kids and they don't understand the complete meaning behind the words they are saying/doing.