As I already said, I've tried old navy. They are shorter (and worse quality) than Gap by at least an inch or two.
Maybe this will help give you a better idea: I wear a size 0/2 and a 35"length (longer with boots or heels). My husband wear a 34x40 in his normal work jeans and can get away with a 38" in dress pants, My 8 year old is already over 5' tall, the just turned 6 year old is about 53- 54" tall and our 3 year old is about 48". The oldest and youngest can get away with slims in better brands because they are a little thicker, but the 6 year old is about a 20 1/2" waist but still athletic - he's starting tailback on his football team and made it to AAU regionals in the 100M dash, so "skinny" jeans are out for him.
See, this is not an easy fix. I'm looking for some brand that I have probably never heard of before. Department store stuff just won't cut it, and trust me - I've looked already anyway.
You may want to check the Jeans on this site: http://www.childrensplace.com/webapp/wcs/stores/servlet/category_10001_10001_-1_135019_babygirl_26601%7C135019_boy%7Cdenim
I have ordered from them in the past. My son also worre a slim and could never find pants that did not gap at the waist and long enough for the legs. I tried some some from the site above and fount that they were better than most Jeans I could get in the stores. The do have a contact phone number where you can speak with someone:
Customer Service1-877-752-2387Monday through Friday:9:00 AM to 8:00 PM ET
Hope this information helps.
JCPenney Online also has some longer lengths online.