Fair =/= equal.  Take the case of 2 children. One studies dance as a hobby (expensive) while the other plays Ultimate (less expensive).  Does the parent pay the same cost for each child which would be equal or pay for what each individual needs to be fair, even if it isn't equal?  I don't begrudge someone earning $20,000 or less for not paying income tax, I pay more than they earn but then I have far more income to live on than they do.  That is fair even if no where near equal.