Your child likes solving things but a thick whodunit still looks like too much. This is the on-ramp: start at the top with a mystery scaled all the way down to a brand-new reader, and each book after it steps up a little in reading level, a few more pages, a slightly longer case, until a real middle-grade puzzler feels like the natural next thing to pick up.
What rises from the first book to the last is simply the reading itself — how much text is on the page and how long a single case runs. Nate the Great cracks a lost-drawing case in a handful of illustrated pages a child can read in one sitting. By the end, The Mysterious Benedict Society is a real 480-page puzzle box that asks a reader to hold a dozen threads at once. Start at the top and let each book feel easy before you climb to the next, because every step was chosen to be barely bigger than the one before it. There's no prize for skipping ahead, and a child who wants to stay on one rung and read the whole series there is doing exactly the right thing.