Cheryl Grimmer went missing on 12 January 1970 during a family day out at the seaside in Australia, when she was three years old.
The British-born toddler was kidnapped from a changing area after spending the morning at the beach with her mother and three brothers near the city of Wollongong in New South Wales.
Since then, police have never been able to find Cheryl, who is presumed dead, or obtain a conviction.
On the 50th anniversary of her disappearance, New South Wales police are offering a reward to $1m (£528,700) for information about the toddler’s abduction that leads to an arrest and a conviction.
Last year, a man, who can not be named for legal reasons due to his age at the time, was set to face a murder trial last year, however key evidence against him was ruled inadmissible and charges were dropped.
A judge said that an interview he gave to police in April 1971 could not be used in court, due to no parent, adult or lawyer being present with him, according to Australian media.
In 2011, a coroner ruled that Cheryl had died, but due to her body never being found was unable to give a cause of death. The working assumption is that she was killed shortly after being abducted from her family – but authorities have kept an open mind.
A woman came forward in 2008 who believed she may have been Cheryl, but a DNA swab proved otherwise.
Cheryl was born in Bristol but moved to Australia with her family in 1969.
Her brother, Ricki Nash, said: “There are no words to describe the pain of losing a sister and the impact Cheryl’s disappearance has had on our entire family.
“Every day we are reminded of the tragic way she was taken from us and we hope this reward is what is needed to bring justice for Cheryl.”
Homicide squad commander, detective superintendent Daniel Doherty, said: “We are appealing to those people who know something but have not previously been inclined to assist police.
“Witnesses at the time reported seeing an unknown male carrying Cheryl towards the car park 50 years ago today, but there has been no trace of her ever since.
“We welcome any information that may assist the investigation. There are now a million reasons to come forward.”