Hello Grace and thank you for your post.
It is really difficult when a child is suffering, I am sorry to hear about your granddaughter. I had it quite severely as a child, particularly on my scalp, so I can empathise. Has she been referred to a dermatologist?
We have a couple of great articles on scalp psoriasis and also psoriasis in children/teens, both have some treatment options that you may be interested in looking at/discussing with your granddaughter's doctor:
https://plaquepsoriasis.com/symptoms-affect-body/scalp-head-hair/
https://plaquepsoriasis.com/psoriasis-affect-health/baby-children-teens-age/
It is tricky with children as often their psoriasis has to be fairly severe for stronger treatments to be considered, although prescription of these medications does occur. Things that I have found helpful for me personally are dovonex scalp solution (a prescription vitamin d derivative), tar shampoos (polytar is good) and also avoiding SLS/sulphates in shampoos. The last one made quite a dramatic difference for me, but of course everyone is different. I hear of people finding coconut oil very beneficial, also.
I hope you find the articles helpful and your granddaughter can get some relief x