Protection is also a theme. Sasuke, even as a child, might have a sense of protecting Sakura from bullies or other threats. In fanfiction, this can be exaggerated and shown in different dangerous situations. And there's the theme of misunderstanding. Their different personalities as children could lead to misunderstandings. For example, Sasuke's quietness might be misinterpreted by Sakura as disinterest, when in fact he cares in his own way.
One more theme is the power of memories. The memories of their childhood together can be a source of comfort and motivation for both of them. In difficult times, they might recall the happy moments of their childhood friendship. For example, when Sakura is feeling down about her progress as a ninja, she might remember how Sasuke used to encourage her during their childhood training sessions, and that gives her the drive to keep going.