[zamadatix]

It may not be the first time the combination of words was ever written but it's certainly the first time it caught on and based on the timings where it caught on from (book then show) https://books.google.com/ngrams/graph?content=sweet+summer+c...

[cinntaile]

It is simply a common phrase to refer to a naive and innocent person, it has nothing to do with Game of Thrones. Although I like how he used it, it immediately makes its meaning clear.

[evan_]

Here's that search without "Sweet": https://books.google.com/ngrams/graph?content=summer+child%2...