Denim comes in different weights so the lighter the oz the lighter the denim. I’ve mainly purchased denim because my children are bigger and I’ve gone to my local Spotlight for this. Having said that though, I have also bought from Fabric.com and there denim is nice to use also.
Denim is a form of cotton (unless it contains spandex) fabric so will soften up in the wash. I don’t do anything special when washing it for the first time other than overlocking (serging) the edges because it tends to fray. I don’t use fabric softener on it and I stick it in the drier in order for it to get used to how it would be washed as a garment.
I think that if you can upcycle your jeans, this is the most effective way of getting the denim you want and if your daughter is still little enough for you to get a pair out for her, that is what I would do.
I think all the pants patterns from O+S look great in denim. I even made a muisc class skirt in denim and that turned out really well. So whatever pattern you choose, it will turn out great.