Three children, believed to be aged one, three and six and their mum Lyndsey Prior, of Worksop, were taken to hospital after a smash on Gateford Road in Worksop at about 4.30pm on Monday.
The incident involved two cars in collision, with one of the vehicles rearing off the road into a hedge.
Two crews from Worksop and a rapid response team from Tuxford attended the scene.
Mum Lyndsey said on social media they were ‘so lucky to be alive’ and thanked everyone who stopped to help.
Firefighter Tim Mallen, who attended the crash, told the Guardian: “The importance of having them correctly fitted saved the children from having serious injuries.
“If they had not been, it would have been a different story.
“You don’t think about what could have happened.”
Mr Mallen said if the seats are not fitted correctly, there is no point in having them at all.
“We host safety days where people can come along and get them fitted,” he added.
The law requires all kids to be carried in child restraints which are appropriate for their size.
Drivers can face a fixed £30 penalty or fine of up to £500 if they don’t comply with regulations
“Statistics show that motorists and their passengers are twice as likely to die in an accident if they do not buckle up,” said Zena Oliver, Nottinghamshire County Council’s principal road safety officer.
“You never know what each journey could bring so it’s vital that everyone wears a seat belt to reduce their chances of death or serious injury in the event of an accident.”
Meanwhile, two people were taken to hospital after a smash in Worksop on Tuesday afternoon.
Police were called to a collision on the Millhouse roundabout, which involved a motorbike and a car at about 2.50pm.
The crash caused long delays in and around the town, with traffic not easing up until about 5pm.