It’s unlikely that the plot, themes, and characters you designed to sustain a novel-length story are a good fit for a chapter book. Chapter books are written for a very specific audience (6- to 10-year-olds transitioning from beginning readers, with a sophistication level far below typical MG fare) and are rarely one-off books like your single title would be. The chapter book market is dominated by series. Look to flesh out your scenes, and consider adding depth to your setting. But don’t pad for the sake of padding, as you may torpedo your pacing and tension. The Newbery Medal-winning The Midwife’s Apprentice is just 22,000 words. Sometimes a short novel is a short novel because it’s supposed to be.