My 3 year old son bought $30 in-app purchase by accident. How do I get a refund?

All purchases are considered final, but you can try contacting iTunes support and explain what happened 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

To try and stop it happening again you can turn off in-app purchases via Settings > General > Restrictions > In-App Purchases 'off' and, depending upon what iOS version that you have, you might also have a Require Password setting on the Restrictions screen which you can set to 'Immediately'

Since the terms of use of itunes and the app store clearly state all sales are final, you may not be able to, but you should contact itunes Support to see about getting it sorted out: https://expresslane.apple.com/Issues.action