Aesop is great and all free https://read.gov/aesop/001.html

It has life lessons, just straightforward things like "don't brag unless you know you're worth it" or "just because they look nice doesn't mean they are". It's challenging enough for them and they'll ask a lot of questions, and they're able to even debate the moral of the story.

They're all really short. Easily 5 mins for today's TikTok attention span. It's an entry level into more interesting books.