I have an almost 7 year old daughter, and her thighs and butt are the biggest part about her. I recently made a pair of art museum pants in a size 7. Because of the elastic back, the fit in the rear seem pretty roomy. I’ve also made her the after school pants several times. I think those would work really well for a child with a smaller waist because it is easy to cinch up the elastic, yet they still tend to have a lowish rise and slimmer fit. And, yes…I have a long love affair with the sketchbook shorts.
I’ve had luck with the brushed french twill (lightweight) and organic cotton twill (much heavier) both from fabric.com. They each are 100% cotton but get where they don’t wrinkle much after the first several washes. Anything with polyester in it won’t wrinkle so bad. I’ve gotten some decent ones from fabricmart.com, but I’m not a fan because synthetics just don’t breathe well in my hot, humid climate.