I've found that even though you are searching for X adults + Y kids according to the display, Marriott's IT system actually processes it as a reservation for X+Y adults. For example, if you try to book a room for 2 adults + 1 child, no matter what it actually says in your search, the reservation confirmation shows 3 adults (and, if applicable, tries to charge you an extra person fee too). As others have said, it almost always works to book just the 2 adults and then contact the hotel to notify them that you'll have a child with you.

Given how long this has persisted, I assume it is a known issue that Marriott simply has not bothered to fix. It's very frustrating when you are trying to make an accurate booking, particularly when it makes room availability disappear like in your case.