Bartek Plachta, eight, from Poland, and Monika Lewandowska-Ritchie, 36, from Durham, were killed following the crash at 14:55 on Monday on the A92, north of the Redhouse Roundabout, Kirkcaldy.
Bartek died shortly after the crash. Ms Lewandowska-Ritchie died in hospital.
Another child, aged nine, suffered serious facial injuries in the crash.
The collision involved a Ford Focus car and an Iveco lorry.
The driver of the lorry and a 36-year-old male front passenger in the car were not injured.
Bartek was a backseat passenger in the car driven by Ms Lewandowska-Ritchie.
The nine-year-old boy was also a backseat passenger in the car.
Bartek's family said: "Bartek was a much loved and treasured son and brother. He was a very popular boy who touched the hearts of many and will be sadly missed by everyone who knew and cared for him."
Ms Lewandowska-Ritchie's family said: "Monika was a loving wife, mother, sister and daughter who was much loved and will be sadly missed by all who knew and cared for her. Our are requesting privacy at this most difficult time."
Police are appealing for witnesses.
Insp Brenda Sinclair, of Police Scotland, said: "Once again, our sympathies are with the families of Bartek and Monika and we will continue to provide them with all the necessary support and they assistance they require.
"Fife Road Policing Unit has been conducting a thorough investigation into the full circumstances surrounding this tragic incident and we remain keen to speak to any motorists who were on the A92 at around 2.55 p.m. on Monday and witnessed the collision.
"In addition, anyone with any further information that can assist with our inquiries should also get in touch."
The families of a woman and boy killed in a Fife crash involving a car and a lorry have paid tribute to them saying they would be "sadly missed".