Just because one group of people can participate in a savings that others can not doesn't make using that savings unfair. You could argue that Disney is providing a plan that doesn't treat everyone equally but your issue would be with Disney.
I don't understand the logic of this. The discussion has been if this use is allowed or not allowed. So far, with little dissent, the opinion seems to be that this is an allowed use of the plan. So using the plan as it is allowed wouldn't be considered cheating or unfair to others. The use of the plan in this manner has no direct impact on people in the situation you described.
It seems that your issue is that Disney has created a plan that allows some folks to have a monetary advantage, pay the child rate for the plan but use those credits for adult meals, and you feel that if they offer that advantage to one group then they should offer it to others. While that is a nice utopian sort of view there are several examples at WDW outside the dinning plan where this type of equity doesn't exist.
For example is it fair that someone that lives in the Florida Keys pays less for an annual pass than someone that lives in Georgia but is closer to Orlando?
Is it fair that there will be families of 4 that purchased packages with dinning for the end of August in January that are unaware they could have rebooked and gotten free dinning? They will be staying in the same hotels as people that payed the same rack rate for a room but got free dinning? Is if fair that they payed over $100 a day for dinning while the family next door got it for free? Shouldn't Disney have notified everyone that already booked to insure everyone should be treated equal?
Is it fair that a family with 2 17 year old teenagers doesn't pay a room surcharge for extra persons but a couple that add an extra adult does?
Setting aside the whole fair / unfair thing if a person uses the plan as it is allowed doesn't make it cheating. If that is cheating or if you are implying that people shouldn't use this feature of the plan because other people can't then I have to assume that in your desire for equitable treatment for all you wouldn't not accept free dinning when it is offered, or any other discount, unless Disney retroactively notified all persons that had already booked travel for that time and offered the discount to them as well. After all anything less wouldn't be fair.