I haven't sewn trousers for DS in a few years so the next problem was finding a pattern. I settled on Simplicity 5362. By measurements DS just made it into the adult Small, but I've been burned by the Big-4 patterns and their sizing before so I attempted to tissue fit on DS.
There are two things to learn from this experience.
1. Don't try to tissue-fit on DS. Have you ever tried getting a teenage boy to stand still for fittings, let alone trouser fitting? I think I'd have better luck doing fittings on the cats; and
2. Don't attempt to make any pattern changes based on said 'fitting' whilst suffering from a fluey-type bug.
However, even with stretching the bejeebers out of the fabric as I did the later rows, I did find the top shrinking to the point where DD can get it on and off, but I expect I'll get some mending to do. Next time I do shirring I think I'll try the zigzag over the elastic method.