Unfortunately the kids sail free is only offered on sailings that DCL is having a hard time filling, and once the KSF is offered they are gone like wildfire since they are capacity controlled. The only advice I can give to get a discount is to book while onboard a Disney cruise, that way you get 10% off of the future cruise you are booking, plus an onboard credit. Never seen any other way to get a discount other that the *GT rates that come out after final payment for a sailing.
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