My daughter has a rash all over her thighs. what could it be?

Too many choices: It would be best to see the doctor for an answer. There are too many possibilities and it would not be possible to tell you without seeing the rash.

RASH: Many possibilities: viral: many types. Allergy: food or outdoor allergens, e.g. Poison ivy, oak, sumac, contact dermatitis. Bacterial: many types. Yeast: monilial diapar rash can extend up or down towards lower extremities. Scabies: can be one cause. Rash: as an association of systemic illness/ rheumatologic diseases. If concerns, worsening, check with your child's primary care pediatrician.