Sales of child car seats have soared in the months leading up to a change in the law, according to retailers.
There is an exemption if there is no room for three car seats
From Monday children under 12 and less than 4ft 5in (135cm) tall have to use a car seat or the driver faces fines from £30 on-the-spot or up to £500 in court.
Some youngsters who have spent years using adult seats will now need to revert to using special child seats.
Safety groups have welcomed the move, which the government says will prevent 2,000 child injuries every year.
Children less than five feet tall and using adult seats risk injury or slipping out of their seat belts in the event of a crash.
CHILD SEAT REGULATIONS
Child seats compulsory until children reach 135 cm or the age of 12
Children up to age of 3 must be carried in appropriate seat
Exemption for over-3s in "unexpected necessity"
Exemption if there are three children but only room for two car seats
Fine could rise to £500
The Royal Society for the Prevention of Accidents (Rospa) believes this is the next step in improving the safety of children in cars.
In a car travelling at 30 mph, children who are not correctly restrained in the back seats are thrown forward with a force that is 30 to 60 times its body weight, says Rospa.
Manufacturers are said to be working round the clock to meet the demand for new car seats.
Halfords has sold double the volume over the last two months compared to the same time last year, while supermarket chain Asda reported a 387% year-on-year surge of all types of car seats.
Tesco said sales of its booster seats - which are suitable for bigger children - were up 1,000% this week, compared to the same time last year.
The chain's spokeswoman Barbara Cadd said: "Sales have doubled and they are still rising.
"We knew there was going to be massive demand for these so we ordered several thousand seats more than we would have done last year."
But there have been concerns that parents have not been properly informed about the new laws, and many will have got rid of their child booster seats years ago.
Exemptions do apply, such as if the car is not big enough to accommodate three seats across the back, then two will be sufficient.
Police in South Yorkshire have been running awareness days to make sure drivers know about the new legislation.
"Initially it will be a softly, softly approach in South Yorkshire with prosecutions following where appropriate," PC John Scruby told BBC News.
"The information's been out for a long time and we've been doing days like this since April this year."