I came here with the same question.
Traveling to europe this summer with my spouse and 18-month-old and the same problem happens with cash or points booking. It is very frustrating and I don't know the right answer. What I think I will do is make the reservation for one adult and one infant (and request a crib) to keep the room occupancy at 2. Are they going to really ask who this other adult is in the lobby and insist that she be added to the reservation?
If there are only 2 on the reservation and 3 people staying, it might impact things like breakfast benefits or whatever