DD who is 4 has had this pretty much since birth on her upper arms, as do I. Hers is very bumpy and red. For a couple of months now, we have been dry skin brushing, and moisturising with virgin coconut oil with tea tree oil. I bought a big tub, put half of it in a screw top jar and added about five drops of tea tree oil. We use this most nights and her skin is so much improved, as is mine. I'm sure if we did it more religiously, and morning and night, it would be almost gone.
I thought I'd post in case this helps anybody else.