The diets of all young children in Scotland deteriorated between the ages of two and five regardless of family income, a University of Edinburgh report found.
Researchers say the finding challenges the idea that a healthy diet is directly linked to income levels.
They found that perceived rather than actual changes in income were more likely to have an effect on the amount of unhealthy food a child eats.
Report author Dr Valeria Skafida, research fellow at the Centre for Population Health Sciences, said: "Changes in how parents felt about money were more strongly linked to their children's diets than their actual incomes.
"This could be because income is not evenly distributed within the home, or because it is perception of poverty rather than measured poverty that determines food choices."
The study, which used data from the Growing Up In Scotland survey, compared the diets of around 3,000 children at the ages of two and five and also tracked the income of their parents over the same period.
It found that at the age of two fewer than one in 10 (8.1%) drank soft drinks more than once a day, rising to three in 10 (28.7%) by the age of five.
One in 20 (6.4%) of the children never ate vegetables at the age of two, but this rate increased nearly five times (27.9%) by the time they turned five.
Researchers concluded this may be because children become more able to demand and reject foods as they grow up.
For parents whose financial situation changed from "feeling comfortable" to "finding it difficult" to cope as their offspring grew from age two to age five, children ate fewer varieties of fruit and vegetables, and ate crisps and sweets more often.
The findings are published in the Journal of Epidemiology and Community Health.