They found for every 10 hours worked the risk of being overweight rose once household income topped £11,000, the International Journal of Obesity said.
Experts said it was a "wake-up call" for the middle classes.
Some 23% of the children in the study were overweight or obese by the age of three.
But the researchers found a link between the risk of obesity and the amount of income earned.
Those earning £22,000 to £33,000 were 10% more likely to be overweight per 10 hours worked than households earning under £11,000.
While those earning over £33,000 were 15% more likely.
However, the researchers also noted that among the working mothers group - 7,500 in total - the risk was 13% and 19% greater.
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