The girl's family have described her as "the light of our lives".
She died from head injuries when rocks fell from a cliff on a beach in North Yorkshire on Wednesday.
Police have said she was visiting Seaton Garth beach in Staithes with her family when the accident happened.
In a statement issued by North Yorkshire Police, Harriet's family said: "We cannot comprehend the enormity of this tragedy.
"Harriet was the light of our lives.
"We ask that you give us time as a family to support each other through our indescribable pain. Thank you."
Emergency services were called to the incident at 4.45pm after reports the girl had been injured.
Despite efforts, she was pronounced dead at the scene.
Officers said they are compiling a report about the circumstances of the death on behalf of the coroner.