The 'I Survived' series is fantastic for getting kids into historical fiction, but the age level really depends on the individual child's sensitivity and reading ability. The books deal with serious, often tragic events like the sinking of the Titanic or the 9/11 attacks, which can be intense. I'd say the core audience is probably grades 3-7, so roughly ages 8-12. The reading level itself is accessible for many third graders, but the emotional maturity needed to process the content is a separate thing.
My own kid picked up 'I Survived the Sinking of the Titanic, 1912' in second grade because he was obsessed with ships. He handled the story fine, but we had some long talks about the reality of the disaster afterward. That's the key, I think—these books are a gateway to discussing real history. They're not just thrillers; they're educational. For a more sensitive 8-year-old, maybe start with one of the natural disaster books, like the Joplin Tornado one, which might feel slightly less emotionally complex than the human-caused historical tragedies. The bottom line is, know your reader and be prepared to chat about the heavy stuff.