The store will ask for your password each time you purchase. But then any purchases made within 15 minutes it will not. So one would assume you purchased something and then gave them your ipad within that time.
There is no option to change this timeout but you can disable the ability to install apps and purchase in-app content. In the settings goto general and restrictions. Select a passcode (do NOT forget this) and disable whatever you want. Its recommended to disable in-app purchases as I have seen many a toddler buy $30 of smurfberries not realising how it impacts their real world pocket.
If you ever need to buy something, just go back in here and disable the restriction. Better safe than sorry.
I do not believe there is any way to get your money back.