Youngsters who get their hands on smartphones or tablets boost the average monthly bill by around £34, the survey found.
More than a quarter of parents are hit by inflated bills because of their tech-savvy children, the data showed.
It found eight-year-olds tended to run up the highest bills, adding an average £59.59, while more than a third (36%) of children aged four and under have made purchases without permission.
The figures were released after five-year-old Danny Kitchen hit the headlines for racking up a £1,700 bill in a few minutes while playing a game on his parents' iPad.
His parents Greg and Sharon, from Bristol, received an unusually high bill after they provided the schoolboy with a password in the belief that he was using a free download.
The latest study showed 28% of parents have fallen victim to unexpected costs for similar reasons. More than eight out of 10 (83%) of these claim they receive an increased monthly bill, while 14% have voiced concerns over affording the costs, and 34% say they now hide their tablets or smartphones.
Meanwhile, more than half (54%) of the parents surveyed said they link their smartphone or tablet to an easily accessible subscription service or direct debit account.
Overall, unauthorised purchases have led to an average increase in monthly bills of £34.18 and an estimated total cost of £30,883,157, according to Windows Phone UK which commissioned the study.