Easily solved. They know how many adults there are. All they need to do is count the email scams and subtract the known number of adults from that number. The remaining number is the number of children. Of course, this assumes that each Nigerian is only involved in one email scam, so my strategy is probably not sound.
With good numbers or bad, what will happen when the census is complete? My guess is nothing at all. They must be scamming some World Bank money for this worthless undertaking.