Complex sentence: Omri thinks it best to send Little Bear, Bright Stars and (with Patrick's agreement) Boone back to their time, and Omri gives his mother the key so he is not tempted to bring them back.
Split into simpler sentences: Omri thinks it best to send Little Bear, Bright Stars and (with Patrick's agreement) Boone. They are sent back to their time and Omri gives his mother the key so he is not tempted to bring them back.