The driving survey, commissioned by insurer GIO, shone a light on the behaviour of 3700 parents, including 600 within NSW, who embark on extended road journeys.

While four in five families give youngsters hand-held computer games, including iPads, and 70 per cent keep treats up their sleeve, 18 per cent of NSW drivers admit to tranquillising their children with drugs that sedate - such as the antihistamine Phenergan - to make the journey more comfortable.

The drug is available over the counter and is commonly used in the treatment of allergies. Phenergan is the product name given to the drug Promethazine. According to patient inforrmation guides, compiled by its various manufacturers, Phenergan should not be used on children aged under two, due to its potential for "fatal respiratory depression".

Those warnings have been in place since 2006 when the US Food and Drug Administration (FDA) identified links between the drug and 22 cases of respiratory depression among infants, seven of which proved fatal. Labels also often state that caution should be applied when the medicine is given to children 2 years of age and older.

Westmead Children's Hospital head of general medicine Joanne Ging told The Sun-Herald: "In terms of sedation, this is an unpredictable drug.

"We would never recommend it for children on plane trips, car trips or wherever, because its side effects can trigger the exact opposite effect, including hallucinations. It's a medication that really should only ever be used for allergies."

Family therapist and child psychologist Michael Hawton labelled the trend as "extraordinary and horrible".  He said: "The whole idea of heading off on a family holiday is to enjoy time together. There's a big difference between entertaining your children and medically straitjacketing them. Kids have a natural sense of meaning and fun. They should be allowed to be children."

One Sydney woman, who spoke on the condition of anonymity, said she was at a one-year-old's birthday party in Bondi last week where a "main topic of conversation" among parents was whether or not to give their children Phenergan on long-haul flights.

Davina Sharry, from Powerful Parenting Australia, said: "As a parent, it's your responsibility to teach your child the skills to cope and behave in situations, including road trips. To reach into the cupboard and resort to drugging that child is morally outrageous."

Mother of two Rachael Moodley, from Earlwood, said she was "shocked" to learn that any parent would drug their child for the "sake of convenience", adding: "What is the world coming to?"

Ms Moodley, like 50 per cent of the population, keeps her children, Raphael and Orlando, entertained with a DVD player. "We use it on long journeys to give ourselves a break," she said, adding: "It means that, for half an hour at least, we're not being bombarded with 50 questions from the back."

But she added: "We make a point of turning it off regularly."

Ms Moodley is also among two-thirds of parents who still participate in the more traditional road-trip games and activities such as I spy and singalongs.

"We play I spy and other observational games, involving car models, registrations and colours, which keeps the whole family involved and entertained. I think bursts of different activities is the key ... which is obviously something you cannot do if you've doped your kids up to the eyeballs."

According to the survey, more than half of drivers nationally confirm they are "sometimes distracted" by their kids with "siblings fighting", "kicking the seat" and "interfering with the driver" among the top niggles.

When all else fails to curb unruly behaviour, 64 per cent of parents nationally threaten to stop the car and leave the children behind. A further 56 per cent said they had also cranked up the music to "drown" the children out.

eduff@fairfaxmedia.com.au