If you have children, you may let them play with your tablet or smart phone. But letting them do so could cost you.

A new survey from CompareCards found that 29 percent of parents have had their children under age 18 use their credit or debit cards without their permission.

Those purchases could be prompted by a seemingly innocent activity: your child's activity on a mobile device. If they click and enter a password, a new app, video game or other feature is theirs — but the bill is all yours.

"A lot of parents can get in trouble because their kid just kind of goes to town," said Matt Schulz, credit expert at CompareCards.

That comes as parents grapple with how to best guide their children with money. The survey found that 52 percent of Americans have let their children ages 18 and younger borrow their credit card or debit card to buy something online.

And 48 percent of respondents said they regretted that decision.

Men were more likely than women to feel that remorse; they were also more likely to say they were caught off guard by unexpected purchases their children made.

CompareCards' online survey, undertaken in October, included 807 adults with children 18 and under.