Children downloading apps
How do I get my money back
Purchases are considered final but you can try contacting iTunes Support and see if they will refund or credit you : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption
If the downloads were done on an iOS device then to try and stop it happening again then you can use Settings > General > Restrictions on it to prevent in-app purchases and to require your password to be entered for every download (you can also hide the App Store app via 'Installing Apps' 'off')