There's an arpeggio that I've heard in different songs and it usually signifies some kind of transition into a "dream world". Here's an example of it, which happens for the first few seconds of Great Fairy's Fountain. The arpeggio that's going on involves a C9 chord. Similarly in dance of the Sugar Plum Fairies there's a predominant B7 chord in their arpeggios, here's an analysis. Note that both songs have the word fairy in them.
I was wondering if it's safe to say that this sound is due to them using seventh and extended chords. And if I wanted to achieve this type of sound does it usually involve arpeggiating some type of seventh/extended chord, basically something more than just a triad?