Ben, from Sheffield, was 21 months old when he disappeared on the Greek island of Kos on 24 July 1991.
A team of 19 South Yorkshire Police officers, forensic specialists and an archaeologist are excavating farmland near to where he was last seen playing.
Members of the Hellenic Rescue Team and Red Cross are assisting.
Ben's family believe he was abducted, but police are now investigating whether he was accidentally run over and killed by a bulldozer.
As the search team began a seventh day of digging, Det Insp Jon Cousins read a statement from Ben's family praising the volunteers.
"We are so incredibly thankful for the help and support of the volunteer search teams working with officers," they said.
"To know that people are giving up their own time and are as desperate as we are to find the answers about what happened to Ben is something we will be eternally grateful for.
"We've been told that volunteers are coming to the site on their days off or straight from work, and we honestly can't thank them enough for that and for their dedication."

The 41-year-old has recorded 696 home runs, putting him fourth on the Major League Baseball all-time list.
He was given a 162-game ban for doping in January 2014, meaning he missed the entire 2014 season.
Rodriguez's final game will be at Yankee Stadium against the Tampa Bay Rays and he will then take on an advisor role at the club.
He helped the Yankees win the World Series in 2009 and was voted the American League's Most Valuable Player in 2003, 2005 and 2007.
"This is a tough day. I love this game and I love this team and today I am saying goodbye to both," Rodriguez said.
"This is also a proud day. I was 18 when I broke into the big leagues and I never thought I would play for 22 years.
"No player ends their career the way they want to, we all want to keep playing forever but it doesn't work that way. Accepting the end gracefully is part of being a professional athlete.
"I want to thank the fans for letting me play the game I love."
The family of missing toddler Ben Needham say they are "eternally grateful" for the volunteers who have been helping with the police search.
[NEXT_CONCEPT]
New York Yankees slugger Alex Rodriguez has announced he will retire from the sport on Friday.