It seems like an "Eleven Years Earlier" thread would not fit into a chapter book, which is short and precise and focuses on one main problem/character goal/plot thread with minor subplots. A chapter book for ages 7-10 would end up around 10,000 words, 15,000 tops- according to my research. How long is your book? While the MC could discover what happened in the past, I don't think a chapter book would have a lot of backstory but it can certainly be woven into a MG . A young MG might have under 30,000 words (25,000-30,000?)
Just my guess here but this sounds more like MG to me.