I kind of have to agree with Mecha Figment "kids are going to whine".
I have been doing the bribe thing in a way. Actually, not so much as a bribe, but reward for good behavior. I don't bribe the "no whining" thing. My kids are saving quarters in their bank for our trip for spending money. When they do their "chores" without being asked - such as make their bed, get up & dressed in the morning without problems, pick up their room, etc - WITHOUT being told or reminded, they are rewarded with quarters. I really think of it more as an allowance and being paid for doing their chores well (but I feel mine are a bit young for an "allowance" so I haven't started calling it that ... don't ask why the name makes a difference).
If they whine or throw a fit over anything, quarters are taken away - and they are HEARTBROKEN when that happens (especially my drama king - yes, I have one too). I actually give them one chance if I hear a snippet of what could become a whine, or a hesitation when I ask them to do something. I reminded them quickly before it escalates "Quarter" (all I need to say - and they know I mean I will take one away) and it stops the behavior.
Their bank is clear so they can see the change building up.
As for whining in the parks. I've decided that if I get too commando with schedules and touring plans, that I am inviting additional whining. They WILL get hot (even in September) and tired, and they may see Mickey and want to stop and see him instead of running to the next ride on my "plan". Enforcing the plan and not allowing them to stop & smell the roses (or shake the characters hands in this case) will increase the chances for a meltdown. This trip is for THEM to experience the magic.
I've planned and planned (and have touring plans in place for the entire trip). But I'm not going to let them run our vacation. I've got 4 ADR's to make and we have to keep those on schedule. I have to work at a conference at CSR 2 days, and MY schedule will have to work around that. But other than that, if I had planned to rush to get FP's to Splash or Space Mountain and they see a cart with Mickey ears ice cream and want it, we will stop and have the Mickey ears ice cream. I really think being flexible about what THEY want to do on THEIR vacation and their first experience with Disney Magic is going to be the key for me to eliminate the inevitable whining that will occur as they get tired (that - and an afternoon nap for everyone

). I will also be more flexible on rules while we are there - especially on the junk food stuff that I normally get pretty strict about. It is VACATION and if they eat 3 "too many" ice creams over the 5 days we have, it won't kill them.