not sure if this is still applicable, as "fashion" changes, but...
my sister was like this (15 years ago). the arizona jeans from penny's were the only thing that consistently fit her. or boys jeans...for two years in high school, her and my younger brother wore the same size (27x36). then again, telling a 12-year old girl to wear boys jeans can bring on a whole new set of issues.
my legs aren't as long, but i really liked gap jeans. i think that they abandoned the best style for that body type a while ago (and so i abandoned shopping there) but the last two years or so they've come back.
For my 11yr old, I took her to get double zero sizes now. She's built much like you described.
Kids jeans just weren't fitting her properly and I wanted to see if she'd fit in the 00's. She did and she feels much better! Hopefully that will work for your daughter too.
Oh and I like jeans to last a bit so I found that even some 0's fit her a little big but nothing a belt can't help and then I don't have to go out buying her more within the next month.lol
Your best bet is going to be LEI, size 0. They run true to size, fairly long and very thin too. That's about the only thing I could wear in high school and I was a twig. I did also wear Levi's because of the waist x inseam too in Middle School.
Now, I wear jeans from Hollister. They're pricey, but they fit great, lean and long. I also love Aeropostale, but they're hit or miss.
Oh, and don't waste your time with Old Navy, Abercrombie or the Gap. Their size 0 fits like a 7, even Abercrombie's 00 fits like a 3.
I am 5'5", very skinny (though I do have a butt, and 34" hip measurement)with a 32" inseam. I've got problems with pants as well, many 0s in women's stores are two big, a 14 in kids great but is too short, a 16 is usually too big.. and sometimes too short as well! Plus I have skinny legs and small waist, so if it fits my hips it bags everywhere else.
is she very tall? Does she need extra-long pants? My sister is 5'10" with 34" hips and a 34" inseam.. I could ask her. I know she has several pairs of 00 pants from American Eagle that fit well, they offer many different inseam lengths.
My 12 yo dd can wear 12 slims at The Gap or Limited Too. We just found her two pair of jeans at Aeropostale in a size 00 short that were a tiny bit big but close enough to buy and wash on hot and dry on high.
She cannot wear regular jeans at all. They literally fall off her if they are anywhere near long enough. She and her 8 yo sister can almost wear the same shorts/skirts....
I agree with others. Gap, Old Navy, Arizona.... their 0's are quite large. Sometimes the slim leg, strechy 00 at American Eagle are pretty small, but then I have other 00 from there that are huge, it just depends on each style. The 00 are only available online. Lei jeans are quite small, also Angel brand which you can find at Ross sometimes are tiny. And Pacific Sunwear definitely. Good luck.
I am very short and thin and have a hard time finding pants that fit me. I buy my jeans at Kohl's and Goody's. I have had good luck at both places finding small waist and short inseam so I'm sure the opposite would be true as well. I used to wear boys jeans when I was a young girl so I agree with trying that as well if your DD doesn't mind. I agree LEI is a good brand for tiny people. They fit true to size, if you want to spend the $$.
I am at the same point with dd, she is 8.5, 5ft tall, but still fits a girl's 8 around. She wore dresses I made for her at 2&3 as shirts this summer! I am making her fall clothes this year - thankfully I can sew!
But I think Target online has a custom order thing where you can put all your measurements and size, but pricey.
So far, my almost-9-year-old has the same problem. She wears slim in all her pants. She's very tall and skinny for how tall she is. I joked with my bf that he should do a morph of her and make her into an alien-