police revealed they are ` vigorously perusing ' paedophile ring theory .
parents of william tyrrell have released a plea for his safe return .
` we ca n't live like this , ' mother said as search entered its seventh month .
police say search of nsw bushland is reaching the conclusion stage .
divers were brought in to search a murky dam in a bush reserve .
william tyrrell vanished from his home in kendall , nsw in september 2014 .