Many devices come with the option of protecting your purchases by requiring a password when a purchase is made. 

This behavior can also often be customized by, for example, removing the ability to make any in-app purchases, requiring a password for every purchase or requiring a password for every purchase outside of a  given time frame.

With an Android based device you can also setup new user profiles and alter the settings for each profile. Parental/payment settings will vary by device and OS, please check the following articles for more details:

If an unauthorized purchase has been made on your account, please locate the purchase details and contact our customer support staff. When submitting your request, please make sure to include as much details as possible, especially date and time of the incident and any purchase receipts you may have received.

It is important that the purchased currency is not spent and remains on the account for removal. Should the currency have been spent, it may not be possible to reverse the transaction.