Your child reads way above their grade, and the problem you keep hitting is that the books at their reading level are written for teenagers. This shelf is the answer: six meaty, challenging, satisfying books that give a strong young reader a real workout while staying written for someone their age. Nothing here reaches for teen material — no romance, no mature themes, none of the older content you're trying to steer around. What these books don't do is pretend the world is simple: a few of them sit with a real hard thing — a loss, a family under strain — but they handle it the way a book for a young child should, with care, and they land on hope. Any one of them fits, so pick by what sounds like their kind of story, not by any order.
These aren't rungs and you don't work through them in order — they're six standalone books that all solve the same problem, which is a young reader who's outrunning their own age group. A Wrinkle in Time hands them real science fiction and big ideas about time and conformity. The Phantom Tollbooth is wall-to-wall wordplay that makes a clever child feel clever. The Vanderbeekers and The Penderwicks are warm, roomy family stories with nothing to brace for. The Wild Robot is a fast, deep read about what makes someone a real person. Front Desk is the most real-world of the group, a brave girl taking on unfairness she's too young to have to face. Pick whichever sounds like your child's kind of story. The whole idea is big reading that's still age-right, so you're choosing by interest, and every one of them delivers a genuine challenge without a thing they're not ready for.