There may be nothing you can do since the terms of the iTunes Store say that all sales are final, and Apple provides tools for you to lock out purchase of in-app items or other content from the iTunes Store to prevent this sort of thing from happening (Settings -> General -> Restrictions on the iPad). But you can ask. Go here:
and follow the instructions to report the issue to the iTunes Store. Pehaps there will be something they can do for you. I would suggest you immediately disable in-app purchases and iTunes and App Purchases to prevent this from occuring again in the future.