Apple must pay tens of millions of dollars in compensation to parents whose children bought unexpected upgrades in apps and games on their iPhones and iPads, as part of a settlement of a complaint with the US Federal Trade Commission.

The FTC said that “at least” tens of thousands of American consumers had been affected by their children spending hundreds or even thousands of dollars in apps such as Tap Pet Hotel, Dragon Story and Tiny Zoo Friends, without Apple obtaining “express, informed consent” from the parents to do so.

To settle the FTC’s complaint, Apple agreed to refund at least $32.5m to parents, with no maximum on the total it could pay out, and to modify how its App Store obtains consent for in-app payments by March 2014. If the refunds do not reach that amount, Apple must pay the remaining balance to the FTC.