I don’t have CAH but I am very sensitive to everything and I can’t wear tight jeans or really tight pants of any kind.  I haven’t worn jeans in years but I recently bought a pair of corduroys that were pretty tight and I had the same problem.  Try khakis.  I know the girls like to wear them tight these days but the khaki material is just more giving.  She could also try skirts or skorts until the irritation goes away.  I also like to wear seamless underwear.   And of course it should be cotton but I don’t think they sell anything else for kids.  Now that spring is here, the lighter weight fabrics should help.