Originally Posted by jameswes

How I've done it in the past (to be clear, not here) is make the booking for two adults on your own.
THEN email the hotel saying something like:
I have a reservation at your hotel (conf#: 123456) and am planning to travel with my child age #. I tried online and it didn't seem to allow two adults and a child in the room. Can the booked room accommodate a child, on a rollaway bed or a crib or something else? If not, I may need to re-evaluate where I'm staying since it is important for us to stay together as a family.

Quick, clear, to the point. Basically you're very interested and already made a booking, but you need some sort of confirmation from the hotel that it's ok.