Suggest Treatment For Rashes On Back Side Of Knee

My 3 1/2 year old has bumpy pimply rash on the backs of her knees, inside her elbows, and under one side of her buttock. They look like very small blisters but don't pop or have fluid. They are sometimes red and she says they sometimes itch. They seem to get worse when she is sweaty from playing outside. Cortisone cream helps somewhat, but they never go away. Our Pediatrician says not to worry, but I am worried!
What is this and will it ever go away?