It is recommended children eat five portions of fruit and vegetables per day -- about 400g.
The Journal of Epidemiology & Community Health study found those who always ate together achieved this, but those who only did sometimes came close.
Watching parents and siblings eat teaches good habits, experts said. This study looked at just under 2,400 children at 52 primary schools in south London.
Parents and fieldworkers compiled food diaries at school and at home, ticking off all the foods and drinks a child had in one 24-hour period.
Parents were also asked questions about their attitudes to fruit and vegetables, such as "On average, how many nights a week does your family eat at a table?" and "Do you cut up fruit and vegetables for your child to eat?"
The study found 656 families said they always ate meals together at a table; 768 sometimes did, while 92 families never did so.
Children in the "always" group ate five portions of fruit and vegetables, compared with 4.6 in the "sometimes" group and 3.3 in the "never."
That equates to the always group eating 125g more fruit and veg, and the sometimes group eating 95g more a day than the never group.
Seeing parents eat fruit and vegetables -- and cutting up portions for children -- both boosted their intake.
The researchers say that, while this study gives a picture of eating habits on one day, it was able to investigate the diets of a large, diverse population.
Meaghan Christian, who conducted the study as part of her PhD, said, "Modern life often prevents the whole family from sitting round the dinner table, but this research shows that even just Sunday lunch round the table can help improve the diets of our families."
She added, "We spend a lot of time looking at interventions at school. But this is showing how important parents are in terms of fruit and vegetable consumption."
Professor Janet Cade, of the University of Leeds' school of food science and nutrition, who supervised the study, said: "Watching the way their parents or siblings eat and the different types of food they eat is pivotal in creating children's own food habits and preferences."
She added, "Since dietary habits are established in childhood, the importance of promoting the family meal needs to be more prominent in public health campaigns."
Azmina Govindji, of the British Dietetic Association, said, "Eating habits developed in childhood die hard, and eating at a table with the family instead of in front of the TV helps reduce chances of mindless eating, which can increase the likelihood of obesity.
"This study reinforces the view that children learn more from what we do than what we say, so it's the role modelling that helps shape their future habits."
Govindji, a practicing dietitian, added, "If children are eating better in childhood, they are more likely to make healthier choices in adult life, and since food directly impacts risks of conditions like heart disease and type 2 diabetes, eating together as a family seems like a small price to pay."