I'll throw my two cents out there as the parent of an "only child" also .....
while I can't say that she would have a problem with a DisneyWorld trip - I will say that she has been wanting to go on the
Disney Cruise.
I think what happens is we try to please ourselves by getting that oh so good feeling from the reaction of "happiness" out of our children. It sounds like you are not getting that feeling and so now you are wondering if you should cave in to her "demands".
I say that since you have this trip planned, you should go ahead and go -- but if its possible - try to pick some things to do that you all normally wouldnt do. I hate to play the devils advocate here but, if you are a budgeter and do the same things over and over when you go - it can become redundant (especially to a child)
Maybe some special ADR? Or go to the Bippidi Bopiddi Botique? Pirates League? Or if your going around a special party like mickeys not so scary halloween party or mickeys very merry christmas party.
Id say make the next trip a cruise -- switch things up a bit.