My 6 year old has hemmorhoid, causes anal itching (checked, no pinworms). Can I use preparation h on her? Will give her a fiber diet, more water too

Yes. The most important thing is to solve the constipation problem that led to the hemorrhoids. High fiber, more water is great. Short term use of preparation h for her symptoms would be fine. Also, make sure she is not afraid to have a bowel movement because it may have been painful in the past - which leads to fecal retention, making the constipation worse. If she is, talk with your doctor.
Yes. The medication is safe to use short term until the changes you suggest begin to make a difference, but it's not something you want to rely on long term. If diet and increased fluids don't do the trick quickly, think about adding MiraLAX (polyethylene glycol) to her therapy for a few months to get things in order more quickly - she already has hemorrhoids, which means this has been to long of a problem already.