If you haven't already checked itout, Eisenberg's Emotion' regulation and moral development review, while a little old, is still a great read. Decety & Svetlova's 2012 review (Putting together phylogenetic and ontogenetic perspectives on empathy) is also pretty useful, albeit not specifically targeted at assessing dynamics solely within that specific age range.
Maybe you could have a look at the following research : https://www.researchgate.net/publication/282760033
that displays, by the way, interesting differences between boys and girls, when empathy development in middle childhood is studied at a relatively "fine grain" level.