that's a tough, no "one size fits all" question, but i'll give my personal opinion/experience:
we had the 4-day cruise, & WDW booked for 9/15/01 (flights & cruise changed).
in the terror of the days prior, we couldn't even think about leaving the safety of home (without going into too many details, DH works at the NYSE & it was a very difficult time).
despite many (unsolicited) opinions saying we shouldn't, we decided that we all needed some normalcy, some retreat from what we went through.
the end result was that our family got some healing, and DH was able to greet Mickey, instead of devastation that following week. the pain didn't go away, but the magic was able to soothe the wounds somewhat.
bottom line is that you should do what you think is best for your family ~ the "real world" & what goes on in it will not go away, but your togetherness as a family can show your children how important they are, as well as showing your own determination not to allow others to control your family's happiness.
(sorry so long, but the "naysayers" were WRONG ~ staying home in fear, with only horrible things to think about, was not in our best interests).
hth,
laurie s