Realize Apple's terms of purchase in the USA are, "All Transactions are final" (https://www.apple.com/legal/internet-services/itunes/us/terms.html), but some countries have different laws so check terms for your country at: http://www.apple.com/legal/internet-services/itunes/. If sales are final then it is Apple's decision if they want to grant a refund. You can always ask, and people do get refunds on a case by case basis, but Apple can also say no.
Get help with an item purchased from the App Store, iTunes Store, or iBooks Store - https://support.apple.com/HT204084
Report a Problem - http://reportaproblem.apple.com - "Sign In for help with iTunes purchases made in the last 90 days"
If that does not work:
Other ways to contact iTunes Support:
https://support.apple.com/contact
https://www.apple.com/emea/support/itunes/contact.html
Use Restrictions on your iPhone, iPad, and iPod touch - https://support.apple.com/HT201304 - "You can use Restrictions, also known as parental controls, to block or limit specific apps and features on your iPhone, iPad, or iPod touch, including access to explicit content in the iTunes Store."
How do I get refunds for purchases that my 3-year old made on my phone?
