My 12 yr old just ordered a in app purchase for $108, how do i stop the purchase?

You can't stop it, you can only ask for a refund.

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"

Other ways to contact iTunes Support:

https://support.apple.com/contact

https://www.apple.com/emea/support/itunes/contact.html

Consider:

Use Restrictions on your iPhone, iPad, and iPod touch = About Restrictions (parental controls) on iPhone, iPad, and iPod touch - https://support.apple.com/HT201304