The sizes in girls clothing can be quite restricting but a parent can find better fits with a little more work. First check out plus sizes in children's. More often then not plus sizes are designed with more room for children. Many girls who wear larger sizes such as your daughter find that by switching to a plus size they wear much smaller sizes. The fact is you are putting her in clothes that are designed for thinner taller children. If you get plus sizes in children's the pants are designed to be larger in the waist and bottom area. In a plus size your daughter would likely wear a 10 or 12 Plus instead of a regular fit 16.
Unfortunately finding plus size clothing for younger children is not always easy. The good news is there are certain stores who cater to this. JC Pennys has a huge line of Plus size clothing for children. Sears has a smaller line but a good selection. Also there is a store called DEBS that caters to younger children who are plus sized.
Another option is to skip the children's sizes completely. Your daughter would find better fits in the Juniors or even better the womens section. You would likely need to find her Petites because she is not as tall as a adult but the sizes are designed better for a women. She will fit womens clothes better then children's and there is a nice selection of things a young girl can wear.
I have a child who wears the plus size and the fit is far better for her then any other. You may also want to check into a new line of clothes for girls called Apple bottoms. These are designed better for younger girls who are larger as well. Fashion Bug also has a great selection of clothing for your young daughter as well.
A different option is to take some of her jeans into a seamstress to have them remove some of the waist and place in some stretch room as well. This hides easily enough below her shirts and will give her more comfort as well. The fee tends to be less then 10.00 per pair of jeans and saves you a bundle with buying new clothes.