I have not seen anywhere that states that when they fell, that Justin landed on top of Ayla. I have only seen that they tripped up the stairs and he fell with her. Her arm could have been outstretched and barely jarred, which could have caused a hairline fracture. Sometimes, fractures are asymptomatic because they are so miniscule. I am a RN & office manager of a busy walk-in-clinic. We see injuries that are completely obvious to everyone that they are fractures based on symptoms & clinical exam, but when we xray them, they are not. We also see injuries where everyone thinks it is a soft-tissue injury based on clinical circumstance and symptoms and then everyone is surprised when the xray shows a fracture.
It seems that everyone has their own picture in their head about this fall & how they landed, but I have not seen it completely described. It is quite possible that Ayla only cried a little bit when it happened and they thought maybe it just scared her. I have not seen it described anywhere that they had any reason to suspect that she had a head injury, based on the fall.
My own opinions, of course.