Two appliances and crews from Broughton and one from Newport Pagnell attended, along with an officer.
Sadly, a boy and a girl died at the scene. A woman and a third child were out of the car with minor injuries when they arrived.
The firefighters provided immediate emergency care, assisted the police and ambulance services, and set up emergency lighting.
Thames Valley Police have appealed for witnesses to the collision or anyone who may have dash-cam footage of the incident.