Since my offspring haven't been eligible for the child's fare in several years my experience is probably out of date, but in those days I had no problem using my Trailpass to obtain the child fare.
After all, you have paid handsomely for that piece of plastic, so even the bureaucrats and nitpickers at SEPTA couldn't really justify saying that you're not a "paying adult". AFAIK that phrase is being used to exempt people who are riding under a free/reduced-fare program like PACE.