Hi Irene,
I have had chronic plaque psoriasis for about 14 years, so I hope that I can offer some advice. However, I am a lot older than 8 years!
From what you say about the extent of your daughter's coverage, it sounds like topical treatments, such as creams and steroids are no longer an option, so UV treatment (is it NBUVB?) is the next logical step, although this will obviously not help with her scalp.
She should get some remission from the UV treatment, but for how long is anyone's guess. What you can do to help is make sure that she is regularly moisturised, and not just on the P but everywhere, but try to avoid paraffin based products before light treatment because they can be reflective and therefore negate some of the benefit of the light treatment.
If you have any specific questions, then just ask and I am pretty sure that someone will answer.
Daryl