First, to prevent this from happening again, turn off "In App purchases" in the Restrictions settings on your iPad. 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:
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:
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). If they refuse, you'll have to provide some sort of payment method to clear the amount you owe. Until you do, you will not be able to download anything further.