Are you looking at US or Int'l? As I understand it all rooms (revenue or award) are based on max of 2 people per room, with possible up charge for additional. However, children typically aren't counted as extra. Most US hotels, in my experience, don't seem to care how many people you show up with.

Internationally, it can be a much bigger deal, depending on country and hotel. If it's US, I'd say try booking it for 2, then calling SPG to change the res to note the additioanl children. At the time of the call, they should be able to tell you if it's ok or if there's an additional charge, etc.

As long as you're not expecting additional beds (cots, etc.) I don't think you should incur any additional charge in the US.