Red and pimply to me sounds like a heat rash, especially if it itches her!!..... Maybe she has had a reaction to something, but if its bothering her itching and sore ect then you can apply calomine lotion to it it will sooth it stop any itching and hopefull cure it!....i shouldnt worry about it aslong as shes feeling well in herself and not poorly! =]
Could it be simply just pimples as a preteen prepubescent thing or maybe she has used a facial cleanser cream that she may be allergic to? could it just be from her sweating and it be like a heat rash? Either way, maybe she should see a doctor who would be able to give you more insight to this problem. Is it anywhere else on her body? Google rash on forehead and see what you come up with.