I think Louise really looks up to Catherine, and may very well feel very 'important' that her older cousin by marriage is taking the time to talk to her. So many times adults dismiss kids, or talk to them as if they don't understand or notice things. I have a feeling Catherine doesn't do that, and since it was such a big day for her Granny, she probably felt a little overwhelmed with all the sights and sounds. Catherine may have been asking her what she liked, and what she was looking forward to. I remember reading that when Louise broke her arm, she was really excited to have Catherine come over and visit with her. I have a feeling that Louise is a little lady in more ways than one, since she's surrounded by so many adults. I'd love to observe her. She's probably such a ray of sunshine and excitement
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