All depends on the property. If the property won't let you book it online, call the property (not the 800#) directly and speak with the GM. Some properties will bend the rules. Others do this deliberately because they really aren't even happy with 4 per room.

Some high end places require one bed per person, but will allow a crib.

Personally, I wouldn't dream of putting 5 people in a room and would book the adjoining (connecting) room for the kids and keep the baby in a crib with the adults.