In a major bit of caveat emptor, the iTunes Store allows an app to be listed as "Free" if the initial download is free, even if the app contains offers/inducements for later in-app purchases.
In-app purchase are anything but free. Typically they are for "jewels" or "brains" or other worthless, overpriced items that are part of some game.
You must place restrictions on the iPad or any other iOS device she will be using. See Understanding iOS Restrictions.