I can’t remember if DEB shops have size 0 (their banner either says 0-24 or 4-24, but my mind is drawing a blank at the moment), but they’d be worth checking out, too.
My sister was wearing a 0 from Old Navy - until she found out that my 13 year old cousin’s size 12 or 14 fits her. Now, she (and my 30 year old cousin) just share clothes with my cousin.
My sister is taller than you - probably 5’3" or 5’4" and she’s only 90lbs so you might find some luck in the girls sizes. I’ve never seen her in Old Navy jeans that look like something a little girl would wear so you might find something good.