Three police forces in England are choosing not to issue fines to drivers who break new laws on child car seats.
Safety campaigners said the law must be enforced
The rules, which came in last September, state children aged under 13 and less than 4ft 5in (1.35m) tall must use booster seats.
But forces in North and South Yorkshire and Greater Manchester said they would not be fining offenders.
Safety campaigners said it was "a matter of life and death" and police must use their powers of enforcement.
A spokesman for North Yorkshire Police said no fixed penalty tickets had been issued since the legislation's introduction, partly because it was impractical to gauge youngsters' height at the roadside.
He said: "We don't have the powers to measure children and request dates of birth. It was particularly aimed at minors, and that was the sticking point."
Their reluctance is shared by colleagues in South Yorkshire, who have also not issued any penalties relating to the offence.
A spokesman said: "Operation staff have taken a conscious decision to avoid fining the public for what is considered a minor infringement.
"We want people to concentrate on the more weighty things. It is about dangerous people, drink-drivers and drug-drivers. In general, patrol people have just been giving advice and it has been pretty well received."
'Less pressing concern'
In Greater Manchester, officers have not prosecuted anybody for failing to comply with the child seat rules - but rejected suggestions they were ignoring the law.
Supt Alan Greene, of the force's road policing unit, said: "The law is extremely difficult to enforce.
"Where a parent has made a genuine attempt to restrain their child but there is no booster seat then we would advise the driver that this is what is needed.
"Our main concern is that people in cars wear seat belts - and if they don't they will be prosecuted. This is a less pressing concern that we offer advice to motorists on."
However, most forces said they would issue fines to drivers caught ignoring the law.
Among those taking a tough line is Cleveland, which has already issued 49 penalties relating to the offence.
A statement from the force said: "Cleveland focuses its activity on children who are unrestrained in vehicles and on the driver who is transporting them.
"We also concentrate on those motorists who persist in allowing children to travel on another passenger's knee."
In Kent, officers have issued 28 penalties relating to the offence.
Derbyshire has given out 17 tickets to drivers without booster seats and penalties have also been issued in Lincolnshire and Nottinghamshire.
A spokesman for West Mercia Police said that a period of grace was given to motorists which ran out in January and now tickets would be issued.
A spokesman for the Royal Society for the Prevention of Accidents said: "Education is an important part of enforcement but ultimately the police need to take action and use their enforcement powers.
"This is not a minor infringement - it can be a matter of life and death."