I believe technically the purchases are final. There isn't a link to a refund page since officially none are given. Still, you can ask:
Apple - Support - iTunes Store - Contact Us - http://www.apple.com/emea/support/itunes/contact.html
iTunes Store: About In-App Purchases - http://support.apple.com/kb/ht4009
iOS: Understanding Restrictions - http://support.apple.com/kb/HT4213 - "You can enable Restrictions, also known as Parental Controls, on an iPhone, iPad, or iPod touch to prevent access to specific features."
Children should not have access to your iTunes account password.
You should also consider removing credit card information from your account so no unconsidered purchases can be made by anybody. iTunes Store: Changing Account Information - http://support.apple.com/kb/HT1918