Oh, I can relate to this one. I have not had luck finding a specific brand, but I am getting better at eying the width in the store and finding ones that look narrow. It will get easier once she is older; then you can find slim sizes, especially slims with adjustable waistbands.
The problem you will face with toddler clothes is that they are all wider because of diapers. My girls both potty trained early, and that, plus the thinness, was a challenge until they got to be about 5 and I could find slim sizes.
My 4 year old is not there yet, but she is tiny, so I can often get away with smaller sized pants with the hems let out. Adjustable waistbands, again, are a must! The other thing you can do with girls is get them lots of tights and play dresses. The waist and hip size is not an issue then. I also buy clothes that are made from shrinkable fabrics, then wash and dry them with lots of heat. This would not work as well if they were not also below average in height.
Hope that helps.