After installing the YouTube for iOS app, simply sign in using your Google account information. Then, proceed to Menu -> Purchases where you will find all of that great video content that you purchased from the Google Play Store. Fire up a video, and you're watching the content that you purchased while using your Android device, on an iDevice.
If you were thinking about doing this reverse, forget it. Videos purchased from iTunes will not play on an Android device, at least not at this point in time.
Use YouTube for iOS to watch your Google Play purchased videos on your iPhone or iPad