So there are three lessons here:
1) WATCH YOUR CHILDREN!! (We never let our daughter out of our sight for a moment and she was required to hold hands with one of us at all times unless we were in a line. No exceptions.
I never let mine out of my sight either, but I actually lost my daughter once. We were on the Family Magic Tour coming out of Philharmagic and a large crowd shoved between my 2 year old and myself and when they finally got out of the way I couldn't find her. I immediately notified my tour guide who took off running in fantasyland and was able to find her in a massive crowd in less than a minute.
So while I am overly cautious and guard my kids with my life, sometimes you can't help what happens. Thank goodness for the system they have set in place to get your kids back quickly!