Other reasons include donating to school charity days and paying for clubs and societies.
And more than a third (34%) of parents do not always pay the money back. A quarter (25%) owe their children £25 or more, while one in 20 (5%) admit to being in debt to their child to the tune of £100 or more.
Fathers tend to take more than mothers and they are also less likely to top their child’s piggy bank back up afterwards, the Nationwide Financial Planning survey found.
One in five (20%) parents admit to pilfering £60 or more annually, while just over a fifth (21%) raid the piggy bank at least twice a month.
Nearly two fifths of (39%) parents said their child had noticed the money had gone missing.
Larry Banda, Nationwide’s director of financial planning, said: “At a time when increasingly fewer people carry cash, we can often get caught short when scrabbling around for change. And quite often it’s children who have more loose change than anyone, as it’s often nicely tucked away in a money box.
“So, it’s no surprise that parents take the opportunity to raid their kid’s piggy bank for things such as parking or the school lunch money.”
He suggested that, as well as having a savings account, parents may want to try to keep a small kitty at home for day-to-day costs when they might need some loose change.
Mr Banda said: “It could save you an awkward moment of explanation should you forget to return your child’s investment.”
Some 2,000 parents took part in the survey.