First off, I enjoy your choice of colors for this piece. You used some dull colors for the girl, but that's good because it provides a nice contrast with the bright butterflies. I'm also glad you didn't paint an extravagant, detailed background; you kept this simple and carefree, adding to the theme of a child's thoughts.
Speaking of the theme, along with the color choice for the girl, I like how her eyes are red around the edges, as if she's been dealing with grief recently. In addition, I've noticed that there are white lines around her hands and neck that look like cracks. I think that's my favorite little detail because it adds to my impression that she's tattered and has gone through a hardship of sorts.
In conclusion, I really enjoy the contrast between the sadness emanating from the girl and the bright butterflies representing her hope and wonder that keep those cracks from breaking.