I have 6 and 8 yo girls who are absolutely obsessed with this story (we are about a third of the way through.)
All I can say is, what a privileged life some of these reviewers have led that their first exposure to family discord was this book. My girls have experienced more than enough to make them hate the topics in this book and yet the strongest emotion they expressed was betrayal at Holly. They want answers why their best buddy betrayed them and flipped on a dime. The inclusion of divorce and resentment was honestly realistic and didn't talk down to them at all.
I would recommend allowing a whole hour for your first read aloud to get through the bleaker beginning. The set up is rough but it picks up the pace and mood soon enough.