Your child can read a short chapter book, but a real middle-grade novel still looks like a wall. This is the bridge: start here, and each book adds a little more text and a little more story than the one before, until a full-length novel feels like the natural next thing to pick up.
What rises from the first book to the last is simply how much text sits on the page and how long the story runs. Clementine is short, funny, and thick with pictures; by the end, The One and Only Ivan is a real 300-page novel that only reads fast because it's told in spare little chapters. Start at the top and let each book feel easy before the next, because every step was chosen to be barely bigger than the one before it. Nobody has to skip ahead, and there's no prize for rushing.