After working in a hotel previously now MOST it's still a request not a gurantee if you read far enough. The reason being is because it's difficult to full sell a property if everyone books the one room type , eventually leading to half booked properties. Most hotels overbook for this reason because there are typically 10% of reservations or more that are cancelled same day or day before. Because they don't gurantee room types it allows flexibility for upgrades , reassignments , etc.
Often when people book rooms they don't even change the number of guests as it defaults at "1" . When we would see this we would assign them to a different room type but then upon arrival they have four people , and it makes things harder but now that not everything is guaranteed we can try to make things work.
Some exceptions are loyalty programs like Marriott where depending on status we will make sure excluding unforeseen circumstances your bed type is met.
As stated making your request known early then again about 3-4 days out is best. Even at check in it's fine to bring up , just try to express it in a nice way - "We are really hoping to get a bunk bed room because my kids are aged x and y " , rather than " You better make sure we have the room we want" . Obviously if you feel the agent or CM isn't listening to your concerns you should always feel confortable asking for the manager on duty.