Dragonspeak: Drew, the Boy Who Talked to Dragons

A cave leads Drew into a magic land. Miss Jane asks him to convince the dragons to let her go to other realms. Drew meets a submarine captain, his giant pet mice, and then some mermaids. He climbs a cliff, and is imprisoned by the Ticals. The dragons tell him Miss Jane wants to conquer all of the realms. He escapes from her, rescues the children she's holding, and they journey to his home.