Ben was 21 months old when he vanished on the Greek island of Kos during a visit to see his grandparents after they moved to the holiday isle.
The toddler was playing outside a farmhouse his grandfather was renovating when he vanished.
In 2016, South Yorkshire Police said detectives believed Ben was killed by a digger which had been clearing land near to the spot where the toddler was last seen.
The digger driver has been identified but is no longer alive and Ben’s body has never been found despite widespread excavation work on Kos.
In a Facebook post to mark the 28th anniversary of Ben’s disappearance on July 24, 1991, his mum, Kerry said: “28 years someone has held a secret. It's never too late to do the right thing.”
She added: “It is now 28 years since Ben disappeared from our lives and we still have no closure.
“We are still in hope that one day someone will come forward and give us the information we so desperately need.”
Her daughter, Leighanna – a mum-of-two who was not born when Ben was alive – added: “An emotional day today watching the girls play in the sun...laughing and smiling, thinking how this day changed my mum’s life forever.
“And 28 years later she still doesn’t have the answers she needs. Answers we all need. “Wherever you are, wherever you may have been, I promise we will find answers.“No mother should be without her child."