I love this scene for so many reasons. It went out of its way to show Kairi feeling an ache and feeling so lonely, home but so far from her closest friends, with no idea when they would be back. She was starting to feel an ache from the events at Castle Oblivion. And she was quieter, more pensive, without kids to play with like the others had.
And someone noticed, and felt concerned for her. Someone saw that, as emotionally strong as Kairi is, she was hurting. Selphie stopped playing and went to her just to see what was wrong. There’s no dialogue, but it felt like a strong connection and a really emotional scene that let us see part of Kairi that we don’t see often. It marked her transition from naive childhood, so to speak.