I'd start with, 'The Magician's Nephew,' but it's not a bad idea.
'The Lion, the Witch and the Wardrobe,' was written for Lewis' god-daughter Lucy Barfield, and is dedicated to her, but, 'The Magician's Nephew,' (Book 6) explains a few little things like the wardrobe and the lamp-post - it was written much later, as Lewis was looking to wind up the series.
'The Horse and his Boy,' (Book 5) is set during the last chapter of, 'The Lion, the Witch and the Wardrobe.'
I strongly recommend that you read, 'The Last Battle,' for yourself before making the decision to read it to a seven-year-old. The ending makes sense, but has invoked controversy since it was written, and many teachers refuse to handle this book - many who dislike the series admit that it's due to the ending.
Spoiler:
"You are - as it is called in the Shadowlands - dead."