In the past I booked the max occupancy and had my Hyatt concierge reach out to the hotel to see what their policy on my 2nd kid. Many times they cannot change in system but would agree to allow the 4th person in the room. I would get written (email) confirmation just in case I ran into issues (which I have yet to - knock on wood). Some hotels could not be flexible with the 2nd kid and I would simply cancel the rez and try elsewhere.