Anita Ramasastry:

Facebook says it will not advertise to children within the app or sell any data it collects to advertisers. There are also, at present, no in-app purchases offered.

Note that they don’t say they won’t collect data at all, which would be something that people who wanted to protect kids would do. Just that they won’t sell it. I’d assume, those kids sign up for proper Facebook later, they release the flood gates on that data.