accidentally purchased an in-app

First, to prevent this from happening again, turn off "In App purchases" in the Restrictions settings on your device. You may also want to turn off the ability to install apps, to prevent purchases in case the child gets hold of your iTunes Store account information, and set the password to be required immediately. For more information, see:

http://support.apple.com/kb/HT4213

As to a refund, that's not automatic since the terms of sale for the iTunes Store state that all sales are final. You can contact the iTunes Store, explain the reason for your request, and ask, though:

http://www.apple.com/support/itunes/contact.html

They're usually pretty lenient in the case of inadvertent purchases by children. No guarantees, though, just as if your child was in a store and ate a bunch of food (in other words, something that can't just be returned).

Good luck.