My LO is tiny with long legs. His shorts are all 18m to 3T, but his pants are around 5T because of length. Presuming you already have the clothes (otherwise I would suggest certain brands that tend to work better and always get jeans with the adjustable waist), if they only need a bit of adjustment, I would sew a piece of elastic a couple inches long on the inner side of the waistband at the side but a bit more to the back, doing one of these on each side. If it needs to come in a lot, I would add elastic starting about at that point at the front of the pelvic bone and going around the back to get to the same point on the other side. Adjust either method to fit with the style of each pant. With either case, when she grows, you can just remove the elastic, or if you add the elastic inside a sleeve and attach the ends with buttons, you can let it out a bit at a time. If none of that makes sense, as it is harder to describe, I can draw a quick sketch and take a photo it.
We had great luck with Oshkosh pants especially. While each style of pant in any brand seems to vary, Oshkosh was hands down the best with almost all fitting, even those without adjustable elastic. With Old Navy, jeans worked because of the built-in adjustable elastic, but their other pants were way too big. LO has a pair of Lee jeans with the adjustable elastic that don't need to be adjusted as much as his other jeans (almost all jeans need to be adjusted by at least 6 buttonholes on each side - also looking for slim cut and boot cut helps to not look like he isn't swimming in them). His other jeans are mostly Wrangler, so probably not as helpful to you. Absolutely look for jeans with the adjustable elastic (have actual working snap and zipper fly) and avoid the ones that have elastic all the way around (pull-on style, though my LO pulls on the ones with the real fly, so they are equaly convenient).
Then Comes Family, LLC is a participant in the Amazon Services LLC Associates Program, an affiliate advertising
program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.