The Department of Transport recently added a check of the towing hitch to the MOT test for cars.
But Juliette Blake, a Derbyshire county councillor, also wants an annual test of road-worthiness for trailers, and has now written to the Government.
It is the latest step in her fight for tougher laws after Finlay Martin was killed by a runaway trailer in 2007.
The youngster, of Old Road, Heage, was walking with his mother, Zoe, 32, when he was hit. She suffered serious injuries.
An investigation after Finlay's death found that the trailer involved had defective brakes, and that a cable which should have activated the brakes when the vehicle became detached from the car was missing.
Trailers are not subject to annual MoT-style tests, but there is a legal requirement to keep them roadworthy. Coun Blake said: "If there was a crash next week in which 60 schoolchildren on a bus on the motorway were killed by a runaway trailer, there would be public outrage.
"We would see Gordon Brown on television, promising to introduce MoTs.
"By that time it would be too late – 60 parents would have to go through the living hell that Zoe and Wayne Martin have to endure for the rest of their lives, thanks to an unroadworthy trailer, which had seen no maintenance in 30 years, smashing into their sweet little four-year-old boy, Finlay."
Since Finlay's death, Coun Blake has collected 1,500 names on a petition calling for an annual compulsory test of roadworthiness for all trailers.
She also got Derbyshire police to carry out a series of roadside checks, including one in July 2008, which found that 80% of trailers stopped were defective. Problems included corrosion, missing breakaway cables, worn towing hitches and defective brakes.
In September 2008, Mrs Blake wrote to Jim Fitzpatrick, the-then Parliamentary undersecretary of state at the Department for Transport, but claimed she had received no reply.
She has now written to the Secretary of State for Transport, Andrew Adonis, demanding changes to the law.
"We need MoTs for trailers full stop," said Mrs Blake, Derbyshire county councillor and Amber Valley borough councillor for Heage, and the leader of Ripley Town Council.
A spokeswoman for the Department of Transport said it was looking at whether a statutory testing regime was needed for light trailers.
She said: "The evidence we have to date suggests most accidents involving light trailers are not caused by maintenance defects on the trailer.
"Many are caused by the way the towing vehicle was driven or occur because the trailer was not connected to the towing vehicle correctly, or was overloaded. These could be rectified by an MoT test"