I don’t have experience making these shorts for kids at the upper end of the size range, but I have made them in knit with good success – that would provide a bit more ease. I think you could also fairly easily just widen the pattern pieces (maybe just the back panel, maybe both) and add a little onto the waistband.
Also again no experience yet with the upper end of the size ranges, but I adore the Art Museum pants as shorts, and the back darts provide some shaping. I might avoid the Class Picnic shorts, but that’s based solely on my experience sewing them for my (at the time) 2 year old – I found them to fit her a little snug, and when I make them again I’ll size up. But again, no experience with the 5-12 range there, so I wouldn’t discount someone else’s recommendation. On the other end of things, I find the Field Trip cargoes to be a generous fit that makes nice shorts, albeit a definite style of shorts that may or may not be what you’re looking for.
