ok...i can't comment on the cruises as our first is in the summer..
but as for europe...
i know lots about that...
i live on the european side of the atlantic now...
and my first trip to europe was when i was 7 years old (on the queen mary no less)...
i loved it at the age of 7, but we spent 3 months in europe and it wasn't exactly a tourist visit...
but i did enjoy it (and remember it to this day.....many many many many years later)....
however i still would advise against the european trip until she's a bit older.....
it's a bit young to enjoy at the age of 6...
unless you were thinking of leaving her at home...
not that there aren't spectacular things for her to do in europe....
however i think she'll enjoy it much more when she's a bit older and can appreciate the incredible beauty of cities like Paris...
i took my kids to paris for the first time when they were 11 and 14.....and they totally adored it.....
of course we did one day in Paris
Disneyland.....very much like Disneyland (but with French people cutting in line.....a culture shock sort of experience).....
then of course, there's the city of science, but your daughter's too young for that....(they have a lot more written in english than they used to but i still had to do a lot of translating of the various exhibits....but as i said, it's geared to junior high/high school more than elementary school)...
and she's far to young to enjoy places like the louvre and the garden of the rodin museum (my kids made us go back to those two places repeatedly)....
there's a nice busch gardens type park in paris....that's called....wait......i'll think of it...Asterix.....it's like six flaggs/kings dominion/cedar point if you're familiar with those....
let's see...
in london she'll adore the dinosaur museum (not the official name but what i call it in my laziness).....and of course, even a six year old enjoys the changing of the guard and all of the royal nonsense...
the netherlands is relatively kid friendly, with some fun theme parks to visit....however, if you have a problem with smoking you'll die there.....actually, in europe in general....no smoking hasn't arrive on these shores....not really anyway....
there are no-smoking signs but that's it....if you ask someone to stop smoking in a no-smoking area they'll get all huffy.....
so if that's a problem for you, be forewarned.....
that was probably the worst problem for my kids...going into restaurants in paris and being asphyxiated by the smoke...
so...
bottom line, if you can postpone the paris/europe trip until she's a bit older i would...
unless you're considering club med in europe...
they have some very lovely family oriented club meds in the french alps....really really nice with tremendous activities for the kids.....although they all speak french which might be a problem for her (though growing up traveling all over the world, it never seemed to be a problem for me.....you get really good at talking in sign language..)
so you might want to consider club med in europe.....either summer or winter....and then with a couple of days in paris just to get your fill of the most beautiful city in the world (ignore the inhabitants...and watch out for pickpockets.....some of the worst i've ever encountered...or rather the best)....
or another alternative is joining up with a family tour of europe....
although i still think those tend to be geared towards teenagers more than 6 year olds....
after saying all that , personally i'd go for the cruise and WDW....