New laws which mean more children must use child car seats are now in force in England, Wales and Scotland.
New child car seat laws have now come into effect
Child seats or booster cushions are now compulsory for under-12s under 135cm (4ft 5in) tall, but consumer group Which? says many parents are not ready.
Drivers face fines from £30 on-the-spot or up to £500 in court if they do not ensure that the children they are carrying comply with the regulations.
In Northern Ireland, the new rules are set to come in by the end of the year.
Retailers have reported soaring sales of child seats, and but supermarket chain Asda has urged ministers to drop VAT on the seats as they are now compulsory.
CHILD SEAT REGULATIONS
Child seats compulsory until children reach 4ft 5in (135 cm) or the age of 12
Children up to age of three must be carried in appropriate seat
Exemption for over-threes in "unexpected necessity"
Exemption if there are three children but only room for two car seats
Fine could rise to £500
There have also been concerns that parents have not been properly informed about the new laws.
Pat Harris, director of the transport pressure group, Belt Up School Kids, said: "Not only is it a short run-up to the actual law changing but also the way it is worded is very confusing for parents.
"Not just parents, but head teachers of schools, also some county councils who are unsure how to apply the new legislation."
Which? researcher George Marshall-Thornhill said: "It's really worrying that so many of the parents we've spoken to don't understand what the law means."
It is also unclear how stringently different police forces will implement the new law from the outset.
The Association of Chief Police Officers in Scotland said enforcement would be governed by "common sense" but urged drivers to be aware of the law.
And Superintendent Phil Davies of South Wales Police said: "We will be targeting schools and shopping areas to educate members of the public about the new legislation."
The onus will be on drivers to ensure they have the correct seats and that they are used.
Birth to 9-12 months:
Rear-facing baby seat, up to 10- 13kg. Group 0
9 months to 4 years: Forward-facing baby seat: 9-18kg. Group 1
4 years to 6 years:
Booster seat, 15kg up to 36kg. Group 2
6 years to 12 years:
Booster seat or cushion, 22-36kg. Group 3
The rules have been updated because most cars now have rear seat belts and without a booster seat, children using adult belts risk neck injuries or slipping out of their restraint in an accident.
Safety groups have welcomed the new regime, which the government says will prevent 2,000 child injuries every year.
Duncan Vernon from the Royal Society for the Prevention of Accidents said the fines were to "reinforce the fact that children do need a suitable restraint in order to carry them safely".
Exemptions do apply, such as if the car is not big enough to accommodate three seats across the back, then two will be sufficient.
And children who are on a short and occasional journey made for reasons of "unexpected necessity" - such as giving a youngster from another family a lift home - will be exempted from having the necessary restraint.
But several police forces have pointed out this will not cover arrangements between some families to ferry each other's children on the daily school run.