Really not need to rush. Rooms won't be available now. Even intercon just book 1 room first. For coach, even walk in may do. but you could just book a session a day before. I've been there 3 times, never have problem.
I think intercon may charge you an adult price for 12 years olf for an extra bed.
For agoda or any other website, look for free cancellation and see carefully last day for free cancellation. Payment usually made before check in for Agoda. Those are standard words meaning agoda don't charge you if you cancel after free cancellation but it's the hotel which will impose the charge. So they mean agoda doesn't charge but it's the hotel.