Your child must be unusually tiny, so perhaps that is true of your child, but it is not true of most children at all. Disney's pricing is not only fair, it is actually rather generous to charge by age rather than height, given the height requirements on WDW rides.
Disney's height restrictions are quite low (max. 48" except for 2 things at DQ that are 51"), and most kids have at least a year's window where they are tall enough to ride everything at the child price. According to charts from the CDC, 50% of US children reach 48" by the time they are 7.5 yrs. old, and even the girls on the lower band are predicted to reach it before they turn 9.
http://www.cdc.gov/nchs/data/nhanes/growthcharts/set1clinical/cj41l022.pdf
My DS was tall enough to ride every single ride in all 4 parks and both waterparks four months after he turned 7. He's not giant by any means, just a bit on the tall side of average for a boy. He'll be 9 at the end of May, and at present, there are only 3 rides at Orlando's major parks that he can't ride for height reasons: Kraken at SeaWorld, and also the Hulk and Dueling Dragons coasters at USIOA. (He is just shy of 53" tall.) I'm sure he will be tall enough for even those by the time he turns 10.