Vietnam's communist government has enforced helmet use for adults for almost one year in the motorcycle-choked nation but the law has not been extended to children and youths riding as passengers.
Yeoh - who acted in the Bond film "Tomorrow Never Dies" and the martial arts saga "Crouching Tiger, Hidden Dragon" - joined the march through Ho Chi Minh City organised by the Asia Injury Prevention (AIP) Foundation.
"Since last year when the law was enacted, overnight 80-90 percent of riders started wearing helmets," she told AFP. "But the kids are wearing baseball caps instead ... The parents need to protect the children.
"Unfortunately, many parents reject helmets for their kids out of a mistaken perception that helmets are unsafe for children."
The march through Vietnam's busiest city was part of a global campaign aiming to cut the world's annual road death toll of 1.3 million people.
"Today, tomorrow and every day, we will see at least 2,000 young children killed or seriously injured on the world's roads," Malaysian-born Yeoh said.
"This is unacceptable, preventable and we have to stop it. We have the vaccines for this disease: helmets, seatbelts, speed enforcement, safe road design. We just need to use them."
At least 12,000 people die on Vietnam's roads every year. The road death rate of 27 fatalities per 100,000 people compares to about 10 per 100,000 in Western Europe, said the AIP Foundation.