What are the dangers of side curtain air bags causing injury?

They have studies that a penger side airbag can decapitate a child...I am wondering of the dangers of curtain air bags, especially in the back seat. My kids either like to put an arm out the window in the back seat or put their head/face slightly out the window or at least even with the window. If an impact caused the bags to go off, what would the extent of the injuries be? Concussion? A broken arm?