..and this is not speaking from experience per say, but, putting myself in both sides of your situation.
first, from the easier side to see, would be yours.
I understand that this may be a one shot only, special vacation.
that being said...the harder part of this, sounds like the kids are a bit inconvienient when it comes to having adult time..(Im not judging, or saying that this is wrong..being a parent I fully understand that trying to have adult time with kids around is basically the same as being 100 percent sure that you definitly bought the winning lottery ticket..and its true....never gonna happen..LOL)
have you thought that maybe getting someone to watch the kids while the ADULTS go away might be another way to do this?is there anyone within the family that would willingly watch over the young ones for the time you go on vacation??
(from personal experience..my inlaws watched over our then 2yr old when we took the rest of the kids...she was just too young to experience the parks we felt, and it made for a great time for our older kids...they got a chance to do alot of stuff with both mom and dad rather than have one parent sit out one thing or another because one of us had to stay with her)
That being said, I know many folks(myself included) feel that not having the kids there would ruin some of those moments(believe me..there were times we wished we had brought the youngest...but, because of that, we're taking her in MAY!!!)
If leaving the kids at home is not do-able..then, you really have to think about as others have posted...what will be this girls true responsibility??
If she is going to be constantly "on call" (at least from reading what you had posted, this sounds like the way your hoping it will be)then the girl would be at the mercy of the kids moods.
(One possible scenario that WILL happen...everyone is having the greatest time on earth...even the kids...UNTIL..one of the kids starts getting very cranky<tired, hot>...will she then have to take all of the children back, or just the cranky one??what happens then...this is really a tough call...because, if she heads back with mr. or mrs. cranky, what happens then if another one of the children start feeling the same way later, and she is back at the hotel??this might put a damper on what happens, and also might cause some problems.
there are other scenarios, similar in nature...I guess my point is, you are going to have to make ALOT of concessions to what is and isnt going to happen, and then your going to have to stick to them.
On the other side of the coin...from the babysitting point of view..I think if I had been given the chance to go to WDW, but with the understanding that I may end up spending more time looking at the inside of a hotel room than actually being in the parks, I would have to say thanks but no thanks.
Even at my age..there would be no way I would accept this..not with someone elses children at least(it would be different if it were my own...anything can happen,and I would rather let the others of my family have a great time and sit with one of the kids if they got sick).
I understand that the girl may see that this is a golden opportunity..but, that also shows some part of how she may be seeing this...more a vacation than a job.
I think you definitely need to draw up some sort of contract, and then, you will have to explain it..if she is on call 24/7...then...you are going to have to let her know that.
As far as that is concerned...IMHO, you would have to pay her for that time as well.
What i mean is this...if you are paying her way into the parks,paying for her lodging, and supplying her food...you would still be responsible for paying her time.The only thing I think you actually could cut back on(or nor pay for) is her food.Afterall, you are taking her to work for you during the entire time.
If you set and establish a particular time frame, then you would still have to pay her for that time.
I know others will disagree, but again, when I put myself in this scenario, it comes down to the simple point of..you are hiring her to work for you...if you want her presence at ALL time, then you are going to have to make those places accessable for her as well...she cant be somewhere if you dont purchase her admission for entry, andshe obviously cant sleep outside either.
Addie, I think you are going to have a larger job ahead of you than you anticipated...just taking along a babysitter isnt going to just work itself out.
Also, as others have already done so...there are many different things that may not have come to cross your mind.....take all the suggestions that have been given from all the posters, and then, go from there.
Afterwards, you may see some light at the end of the tunnel...but...
I hope whatever happens, it works out well for all concerned.
(One last thing I would point out...nothing would be worse than having your vacation time ruined by misgivings on anyones part...and if as you stated this girl is the wonderful babysitter you say she is, then it would be horrible to lose her because she felt taken advantage of..PLEASE...let her know EXACTLY what she is getting herself into. that way....you and she know what to expect...and, one other thing....the other family(I believe your Brother and SIL???make sure THEY understand her role as well....I cant imagine anything worse than being told I am to do this or that by someone else when that is not part of the agreed upon employment.)
Whatever happens..have a great vacation!