Urticaria (hives). I had exactly the same looking thing when I was bitten by a spider. Unfortunately for me, in addition to the hives, I also had angioedema (swelling - my eyes even swelled shut - it looked like someone took a bat to me). DH took me to the doctor right away as I was pregnant at the time, but the hives and the swelling all went away in a matter of days. We figured it was from a spider bite, because the day before I broke out I found 3 or 4 spiders in my office. I didn't even feel the bite.
Hives can also be caused by a virus, especially in kids. Has she been feeling okay other than the itching from the hives? Are there any viruses going around your neck of the woods? I know here in MA strep throat and some sort of flu has been wreaking havoc the last couple of weeks. They even closed my 2nd DD's school because so many kids/staff were sick.
I hope she's feeling better soon, poor thing, I remember how itchy it was. I know one thing that helped with my itching was an oatmeal bath (I think it was Aveeno that I used. They come in single bath size packets). Good luck, and again, I hope she's feeling better soon!