We pretty much buy all of our jeans from Old Navy and love them. They hold up really well, and they have adjustable waist. My 3 skinny little guys need that! I try to buy each kid a pair or two during their $10 sale. I just placed 2 separate orders, using 2 different $10 off codes (use 2 separate email address to sign up for emails to get the codes). I arranged my cart to get them right at $50 for each order to get the $10 off plus the free shipping, then it was $40 plus tax after that. Make sure you choose free shipping before going on to the checkout b/c it seems to select the $7 shipping by default. So, basically, I got 20% off plus free shipping. They also have uniforms on sale.
As for size comparison, I think they run pretty true to size. However, I will say that my oldest daughter and I order the "short" length in ladies, and they are still too long for us. BUT they are still shorter than most "short" lengths in other brands, which is why we get our jeans there. We're both 5'2". For kids, I think they're probably true to size. My 11yo daughter is almost 5'3" and wears a size 14 in girls, but a sz 2 in womens, short length as well.
Anyway, hope that helps!