I have a few apps that allow in-app purchases (Android, not Apple) and it's pretty clear that you are making a purchase. That said, it would not be at all clear to a kid...and why would they care anyway?
Words With Friends and Draw Something both have in-app purchases available for extra features, neither mention that in their description in the Market (Play).
There are plenty of free apps that provide paid services, this is nothing new. The app for Amazon MP3's is free but the MP3's aren't. The app for Home Depot is free but the kitchen sink is not.
It could probably be made clearer in the app descriptions but, I believe, it still comes back to the parents (in this case) and end user. Don't push the "buy" button if you aren't planning on getting a bill. Don't hand your kid a phone and tell them to have at it unless you are planning to monitor what they are doing. Don't plan on getting off scot-free by suing someone for your lack of supervision.
Yes, it is a bit of a gray area but it's up to you to be diligent, especially when your credit card and kids collide.