What Causes Raised Small Vesicles On Hand Of A 4 Year Old?

My 4 year old grand daughter who lives with me has developed a series of raised small vesicle like linear patterned raised areas on the back of her hand going towards her elbow. Her sister has one on her forehead and one on her upper arm. Any idea what they could be? The children are not scratching, but the 4 year old's bumps increased from about 4 marks on hand to at least 30 going up her arm within 24 hrs. She has lived with me seven months and has not had this pattern in the past.