my experience with stitches designed for knits is that they create a very bulky seam line because of the multiple stitches and the holes made by the needle going in and out of the same hole several times can be a problem on some fabric. I agree with a previous poster who described sewing knits with a straight stitch or even a small zig-zag.

reference your question about how much stretch is in the fabric and what the pattern requires. If you look at the pattern it usually has a small bar up the side of the back, or maybe across the flap and says something like "this much fabric should stretch to here" and gives you lines to use to check your fabric. The bolts will not tell you how much fabric, usually -= you need to check it against the pattern "stretch line", or be able to calculate quickly how much percentage the stretch is. Always check the stretch across the fabric, not along the selvedge.

Like several others, I took Stretch and Sew classes and still use their techniques...

there are ballpoint needles to use on knit do you gon't get holes

I've made so many knit pants for myself and for my GS's, but I NEVER use anything other than the straight stitch or the knit stitch. I just gave the fabric a little pressure and that worked fine. You don't want to pull very much., in fact, I don't pull . Also, use the ballpoint needles. I really like the McCall patterns best, but I use all kinds.

Think that is the main problem with the jeans. She is so long waited that when she sits it pinches in all the horrible places. She is tall but because of her long waist her legs seem to short for the size she needs. I end up cutting off 3 or 4 inches on anything we buy.
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I buy Petite sizes mainly for that long waist I have, even though I am not too short (5'3+) and not in the least petite in size, BUT I do get a good fit from waist to crotch. In dresses I find that the belt line is right under my breast line, and that's not any good except for the old styles that only look good on pregnant ladies.

Forgot to ask her age, but if she's old enough to worry about the fit, then she's old enough to want a good looking pair of whatever it is you're going to make for her.
Have you ever made her jeans from scratch? Burda patterns have some neat ones, plus, if you should ever get her measured by an expert you could make her jeans that actually fit her and fit accurately.