Can it fit an 8 year old?

i'm 26yrs old, but at target i can usually fit into girls size 10-12 and sometimes even 7-8 in shorts/skirts, so i'm hoping my answer will help a little ! i'm only 5'2," 90lbs, (size 22 or 23 in jeans) and i purchased an XS. they fit, but i have to pull the drawstring as tight as possible and the shorts underneath are a little baggy on the thigh. that being said, i'm not having to constantly pull them up or anything unless my phone is in the shorts pocket (since it weighs them down). they fit well enough that i'm going to buy more colors and i love them, but i definitley also own athletic shorts that fit better than these too ! if your child is around my size or smaller, the XS might be too big but if they're my size or bigger, i think it's worth trying !

My waist is about 27 inches and my hips are roughly 32 inches wide. What size would fit me best? (Xs or small)

I bought these for my twin granddaughter which are 11 I got them x small fit them good you should probably get the small.

Are these shorts decently long?

Yes, they are a little longer than the flowy part of the shorts

Could I wear my diaper under these shorts?

I’m sure you could, although the legs are open and flowy. I purchased these for my granddaughter and they are short, but the short under the flowy part is a little more fitting , so they don’t show your bottom.