I'm a nanny for a 14-month-old girl. Today her mom went in to deliver her little brother (yeah, we're gonna have our hands full!) Because she left while I was still on duty, I ended up being designated as "person-to-watch-the-big-sister-until-Mom-and/or-Dad-gets-back." I've been this person before, and it always makes me feel good for some reason. DH and I took Sasha out, and gave her some junk food - shh, don't tell Mom and Dad! Then we went on a long walk to bring her home late and tired. It seemed to work, because she fell asleep without too much fuss. Now I'm just sitting here waiting for the phone call...
I'm sure others here have had that duty at some point.