Complex sentence: They then attempt to return Bruno to his parents and get them to flee the hotel in case things go wrong, but they refuse to believe Helga's story about Bruno being turned into a mouse and declare her to be insane.
Split into simpler sentences: They then attempt to return Bruno to his parents and get them to flee the hotel in case things go wrong. However, Bruno's parents refuse to believe Helga's story about Bruno being turned into a mouse and declare her to be insane.