What is FAIR is not always the SAME thing for everyone. At the beginning of the school year, my class has a meeting and we generate our class rules. We call these "agreements" (and sometimes for little kids I call them "promises.") The students agree to follow the rules we generate. Then my assistant and I make agreements with the class too. One of our most important agreements is to help make things fair.
I find the concept of fairness to be one that even my youngest three year olds could understand. When the adults promise to be fair, it sets the stage nicely for differentiating your lessons according to your data. When someone asks me why something is different for one person or one group, I simply reply "Because I promised you all that I would help make things fair; and you have what you need right now, and so do your friends."
Clearly the boy who is tall doesn't need a box to stand on (even if he wants one) and clearly the child who is the shortest needs 2 boxes if he is going to be able to see the game. What is fair, is not always the same thing for everyone.