Yes, you can call housekeeping for a rollaway bed if you don't get a daybed. Do NOT call the front desk, call housekeeping. If they should tell you there is a fee, just go to the concierge desk on five and tell them you know there is no fee for the bed and would they please take care of it. I've heard people say they are told there is a fee and there is not a fee!!
Make sure your ONLY request on your reservation is "day bed please ... two older children who do not want to sleep together".
When you get checked in, ask immediately if your room has a daybed. If it does not, ask them if they will please move you to one. If they do not have one available on the day you check-in, ask if one is available the next day and (if you want to), set it up for a room change the next day.