I seriously don't mean to be obvious or anything but you do realize if you buy local you can take them along and try the clothes on them right? They should also have slims.
Penney's and even Walmart often carry the khaki's and polo's at this time of year so they can help out the families that send their kids to private school.
I can't imagine ordering anything online that I didn't have time to reorder and send back and forth several times.
OR your other option is to buy several sizes and just find what fits, then send the unwanted clothes back and order more of the ones that fit. Seems like a lot of work for a time costly activity.
I can tell you that my granddaughter is 8, is about 56 inches tall, weighs about the same as her height, and is just now going from a 6 slim to a 7 slim. I would never buy her anything that she can't try on due to the hassle of having to pay return postage and going back and forth.