You want your child to love the old classics, but handing a modern reader a hundred-year-old novel cold is how you put them off for good. This is the on-ramp: start here, with the most accessible classic there is, and each book asks a little more of them than the last, until the dense, old-fashioned prose of a real doorstopper feels like a place they already know how to be.
What climbs from the first book to the last is simply how hard the prose is to read. Charlotte's Web is a classic that doesn't feel like one, plain and warm and modern in its voice. By the end, The Hobbit is a full fantasy novel with invented songs, maps, and dense, unhurried Tolkien sentences. In between, the language gets gradually older and thicker, the Edwardian slow-burn of a walled garden, Anne's page-long speeches, the cozy density of the riverbank, the nineteenth-century sermonizing around the March sisters. Start at the top and let each book feel easy before the next. Every step was chosen to ask just a bit more than the one before, so nobody has to skip ahead and there's no prize for rushing. Several of these are older books that carry the dated language and attitudes of their day; those are named honestly in the heads-up on each.