Lyric discussion by MingusMingus 

Cover art for The Summerhouse lyrics by Divine Comedy, The

The song is about growing up. For most of the time it seems like it's about innocent childhood fun (climbing trees, sunny days, picnics, swimming naked) but the line about 'new games' changes things.

It seems suggestive. 'Found new games to play'. The two young characters have had fun playing kid's games but now they are discovering physical pleasure.

The only thing that makes me hesitate with this interpretation is the characters are so young in the song ('you were only 9 years old and I was barely 10'), but maybe they were precocious? Anyway the song's narrator sees the games as a sign that the boy and the girl were both growing up. And leaving childhood behind.

[Edit: Comment was too long, got rid of repetitive stuff]

My Interpretation