My teenage daughter is quite slender. Her backpack has been fitted by the wonderful staff at REI but after even a single 6 mile hike her iliac crests are quite sore. Has anyone added padding to that area of the belt if so, recommendations in types of padding?
It is unlikely that you will be able to improvise padding that will fix the problem satisfactorily. To work properly it really has to be integral to the pack's belt. I would suggest trying a different backpack. Different brands and models fit in different ways so even if you got the the fit right in the store, in practice the pack may not work. Sometime the first pass on fit turns out not to be right possibly because you are not used to the pack and don't know it feels wrong. If you are between or on the edge of sizes fitting can be challenging.
Possibly the issue may have to do with the amount of weight being carried. As a rule of thumb you want to aim at a maximum of 20% body weight which can be a challenging to keep under for smaller and slimmer people. It is only a rough guide but keeping your carried gear purposeful and weight efficient is important for backpacking comfort. Different packs are also better at carrying more weight than others. Take all the carried gear with you and add the weight for the supplies when trying for another pack. Get advice on carrying less or using lighter equipment.
It is also possible that the problem has to do with not being used to carrying a pack like this and you need to build up some tolerance. Some additional "practice" might fix the problem. Shorter hikes carrying less weight making minor adjustments to the fit might be worth trying if revisiting the purchase isn't immediately practical.
To add to what @OldGuyot wrote, I had a bit of soreness too when I first started carrying a pack with weight in it.
I changed what I was wearing and that made a huge difference. I was wearing hiking pants with a belt and the hip belt was pushing my pants belt into me a bit and causing that soreness.
I switched to a pair of drawstring hiking pants (and the string lies flat) and that has helped immensely. So I would also look at what she's wearing
Thumbs up to both comments in this thread. I always have problems with soreness on my hips. I am a custom pack fitter, at least I was years ago. My pack is fit correctly and I am very cautious about clothing between my hip belt and my hips. Some of us are gifted to be sensitive I guess.
By the end of the summer I can tolerate more, but at the beginning it's a struggle.
I managed in the outdoor sporting goods world. We had a comment about some versions of complaints that people would give us.
For some people the best sport is perhaps bowling.