My mother, father, and gf are actually all making the trip down with me though I don't believe any of the three will be attending any of the events, they basically just want to see the campus and the town (and get to travel a little bit South and enjoy what is most likely going to be nicer weather). My father never attended college (went straight from HS to a steel mill) and really loves getting to tour schools and see where his children are attending. He attended a Case Western ASD with my sister about 7 years ago and sat in a class with her-- to this day he still talks about how much he enjoyed it.
From what I've gathered its completely acceptable to bring a guest if you so desire. I like the fact that I'll have another perspective on the school from people who know me well. Like I said before I'll most likely be attending the events by myself while my family checks out Lexington, but I expect to see a lot of people with guests at the functions.
When I toured WUSTL last week one of the other admitted students taking the tour who sat in on the class I sat in on had his mother with him, and it didn't seem like anybody thought it was out of the ordinary.