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 family of missing toddler Ben Needham say they are "eternally grateful" for the volunteers who have been helping with the police search.