help on clothes shopping for a tall slim 12 yo

[beckyvicky]

ds is 12, v.tall and slim and we have trouble buying trousers.They are either too short by miles, or if the correct length, they fall off as soon as she breathes.
Some Next trousers with adjustable waists are OK-ish and Zara kids were a revelation (age 9-ish fit and are long enough)
She's not really an adult size 6 petite in Next, waists are still too big - any ideas where to find girls trousers to fit a non-standard child-size, or even, in adult sizes, where would I find size 4 (I have never had to shop for size 4 in my life before!)
At the moment she wears a lot of skirts or cut off trousers, or standard trousers that look like cut-offs.

My 12 year old is tall and slim (like her mother )

We shop in Asda, they have a teenage range. Mark One and New Look, and also Top Shop who do tiny sizes and longer legs.

Gap do size 4. DD also found some tiny size 6 jeans in Debenhams. She is just 13, v slim and about 5'5".

Most of her clothes come from Gap, Zara (including the teenagey type section in the ladies dept) and Topshop (although v expensive methinks).

My DD is nearly 11 and very tall and thin.

Gap used to do 's' range for slim kids, we were always able to pick up bits and bobs in the outlet here so it obviously didn't sell well in the ordinary shops! but haven't seen anything for a while so maybe its discontinued?

Asda's teenage range goes down to size zero and we have found some nice cargo pants in fat face recently.

she's 5'3 and growing
thanks for all the help so far