Try the Arizona jeans from JCPenney. They work well for my long-legged, skinny boy (older though--size 6) and my not quite as long-legged but still skinny girl (size 3T now). They go on sale pretty frequently. Some people have trouble with the knees not lasting as long as other brands. IDK because we don't spend $$$ on the other brands, so I can't compare.
Sometimes Circo and Cherokee (Target brands) work well for DD; other times not so much. But Target has always been great with returns. (We keep all receipts.)
DS '04 "Boogaboo"
DD '08 "Lilybear"