Seriously, your daughter is 18. You don't have to baby sit her. Give her some freedom and save yourself some money and get her a room across the hall. What is she going to do? Seriously, she is going to dance, eat, and meet new friends. If she is in the room with a friend it will be even better. You can set some ground rules; like a curfew, etc. Do you really want to have a teenager running in and out of your room.
I am assuming the 2 year old will be with you.