Back in 2008 I wanted to get the family passes and I wanted to get them at the lowest price of the season in the fall for the 2009 season. My oldest was just under 48" and I was curious of this exact situation. I called guest services and I was told that what hieght they are at point of purchase is what will hold true for the season of the pass being purchased. Sure enough the 2009 season my son was over 48" and over 50" by the end of the season. Not once were we ever stopped for his height and we went over a dozen times that season.
Couple of things, I believe, that you pay one price at point of sale. You shouldn't have to worry about getting caught and paying more later down the line. You paid the price with no intention of deceit. These are kids we are talking about, good business practices will go along way in that customer returning. Also, if they do ask you nicely to upgrade to adult, is that price based on current prices or when your orginal purchase was made? Either way it is good business to honor the original contract, at original point of sale.